1. Learn a language
This can help you put yourself in the shoes of your future students and get some ideas on what ways you like learning and therefore are most likely to choose when teaching.
If possible, try studying in a monolingual class (one where English isn’t used), learn to use phonetic symbols in a dictionary (languages where the spelling is irregular like French are good for this), and learn as much grammar terminology as you can.
Learning another script like Thai or Russian can also be good experience. Keeping a language learning diary and experimenting with as many self-study methods as you can (e.g. different ways of learning vocabulary) will also be useful. If you can’t find or can’t afford classes, a conversation exchange or self-study materials are nearly as useful.
2. Buy and use an Advanced learners’ dictionary
This can be a difficult habit to get into, as native speakers naturally use skills of guessing new words from context (skills that you will be trying to teach your students) rather than reaching for a dictionary all the time.
Still, try to keep a good Advanced level learners’ dictionary around and keep it in mind when reading something pretentious or playing Scrabble, because even if you don’t use it before the course to improve your own vocabulary or pronunciation, you will certainly need it during the course to help you explain the meaning or pron of words and expressions to your students.
3. Learn some grammar
Learning a foreign language can be an interesting way of doing this. Otherwise, people tend to find that analysing sentences for parts of speech, tenses etc. is the most interesting thing to do on your own, especially if you are a mathematical/ analytical kind of person.
Recording and transcribing one of your own conversations and then analysing the grammar (and lack of grammar) in it can also be more motivating than most grammatical study.
Other interesting topics include regional differences and differences between spoken and written grammar, e.g. looking at a corpus based grammar like the Cambridge Grammar of English and seeing how much you agree with its findings.
4. Mix with foreign people
This should help you work on speaking slower and with simpler language. Ways of finding people to speak to include conversation exchanges and volunteering to help foreign people, e.g. as a tour guide, host family or in a local tourist information office.
If there aren’t any people from other countries to practice on, time spent with a hard of hearing relative or small children might be a good second best.
5. Get yourself interested in the language
Having a detailed knowledge of the history of English is in no way useful in the classroom, and can in fact cause all kinds of confusion if you try to introduce your students to it. Prompting your own interest in such things can, however, improve your enthusiasm for your subject, which is perhaps the most important quality for any kind of teacher.
Many famous writers have written books about the English language, but Bill Bryson’s Mother Tongue is almost certainly the most entertaining. Writers from the world of linguistics who are also very readable include David Crystal and Stephen Pinker.
6. Polish up your own English
Although factors like speaking speed and not having a good lesson plan or not sticking it have a lot more impact on passing a TEFL certificate than your ability not to make spelling and grammar mistakes, having a mistake pointed out by a student is one of the most embarrassing things that can happen in class.
Ways of working on this include keeping lists of mistakes you make that a spellchecker picks up and reading books on common mistakes. Reading a book on common mistakes designed for non-native speakers can also help you become aware of which mistakes to look out for when your students are speaking and writing.
Alternatively, a book written for native speakers, especially one written in a lively style such as Troublesome Words by Bill Bryson, is more likely to focus on the things you have problems with. Just as for your students in their grammar studies, you are probably better off using such books in small chunks with lots of breaks.
7. Learn the phonemic script
Like certain types of grammar, learning the secret code of the phonemic script is something that mathematical/ analytical types get into straight away. For others, it is reassuring to know that you are not expected to be able to write whole sentences out in phonemics without a dictionary even at the end of the course.
Interesting ways of getting into and practicing this include learning foreign languages with the help of a dictionary, looking up words you have never known how to pronounce (e.g. foreign words in English), and looking at phonetics more generally and using it to try and pronounce sounds that don’t exist in your language.
8. Learn the jargon
You should quickly pick this up when you are on the course, but it can be daunting until you do. Easy ways into it include reading forums and blogs by people who are already teaching English, where they will often use expressions such as TTT and Find Someone Who without any further explanation.
You can then look these things up in an English teaching book or elsewhere on the internet, or just leave a comment asking them to explain.
9. Teach something else
Although teaching languages is very different from teaching maths, the same patience, ability to explain in simple terms etc. are necessary for both. If you can’t get a paid or voluntary teaching or assistant teacher position, just helping out at a summer camp or with someone’s homework are useful too.
10. Study something else
This will help you prepare for the more academic parts of the course, make sure you have all the stationery etc you will need when you start writing essays, and help you give your students tips on self-study skills such as finding time to do homework.
This can be a way of making up for not having anything to study if you want to get yourself into the habit of sitting down to do something everyday, or as a way of realising what your own problems with English are. The simplest things to get into writing are a diary or a blog. If you are lost for things to write about, your struggles to prepare for the CELTA might be a good topic.
Your course provider will probably give you a list of books to buy, but please be aware that this might not be the same as a pre-reading list. For example, Michael Swan’s book is hardly meant to be read from cover to cover and The Practice of English Teaching makes little sense until you have actually been in the classroom and so can start to make your own mind up about the many options it gives you.
If the place where you will be doing your cert doesn’t provide a specific pre-reading list, you might be able to find one elsewhere on the internet or can make your own by looking for books that are simpler (How to Teach English rather than or before the other Harmer mentioned above) or more systematic (a grammar book that is arranged logically by sections rather than A to Z).
13. Get organised
One of the greatest challenges of the CELTA is just lack of time- time to do your coursework, time to relax and time to do anything else that comes up in your life. You can make more time for all of these things by doing the things mentioned in the other points above before the course starts, but also by buying all the books and stationery that you could possibly need, clearing your email inbox, paying all your bills etc before the first day of the cert.
You may also want to put some time and effort into improving your time management skills by reading a book on it, filing things that are often difficult to find, starting a to do list, buying a new diary etc.
14. Learn timing
Planning how long things are going to take and then checking how long they really took is not only another good time management method that can help you deal with the pressures of the course, but is also a useful skill to have when writing a lesson plan and checking your progress through it in class without having to look at your watch every ten seconds. You can practice this with literally any daily task.
The thing you will have most problems finding time for during the course is relaxation, so when you are reading up for the course do so in a reclining chair with a freshly squeezed orange juice just before you go to the pool, so that you start the course healthy in body and mind rather than already feeling frazzled.