1) Network, like crazy!
Talking to other assistants, getting your name around, building a website, a facebook page, twitter...networking on and off line is key to developing your name and yourself as a well known local and even global photographer. Hello deviantART!
2) Be Professional
Dress smartly, but not too smartly (gauge the job). Keep your phone away (it's not your camera on this occasion) and be polite. You're an assistant, but you need to act like the main photographer.
3) Learn 'on the job' as well as from the 'book.'
Learning Photography at College or Uni is great. But unless you put theories and motions into practice, you're not going to cut it. Don't do everything by the book, but remember that the book is there if you need it. Photography, true pure honest and emotive Photography, cannot be learned in a classroom.
4) Research your Photographer
There's nothing worse than turning up to support a photographer at an event and having no clue about their style. The most important thing to find out is whether they're candid or posed. Whether they are journalistic or not. Whether they're both! Find out their style before an event, and even research them before an interview. Knowing their favoured shooting subjects is always helpful. And if you can talk about their work too, a favourite piece that you might have seen, even better. Just don't sound creepy.
5) Check your mentality
A while ago I read an article from a well known Photographer here on dA suggesting that you're a failure from day one if you offer yourself, your photography skills, free of charge. It's not true. At least, in my experience anyway. Starting off as an assistant is the way to get your foot in the door, get a portfolio started, and get your name around. People will remember you, they'll remember you if you make a good impression. If you don't want to do a job for free, then make it affordable for your client, but keep in mind your experience and skill base.
6) Don't be a spectator
You're not at an event to gawp at Granny showing her knickers after one too many drinks. Nor are you there to get all tearful and sniffly when the bride and groom exchange vows. But you are there to capture that emotion (even in the granny scenario) on camera. Don't forget yourself, don't forget what you're there for and if like me, you're particularly emotional at such events - try and avoid being the main photographer or assistant at an event being held by a family or friend. Before you know it, it's over...and you have no shots.
7) Use your Initiative
This is applicable during an event, and when finding one. Put yourself out there. Don't sit around waiting for the jobs to come to you, look for them, ask, offer, help, promote...and so on.
8) Get yourself a decent business card
Seriously, these things can be incredibly cheap these days. Get yourself one. Don't steal the photographers limelight at an event, check it's okay first, but if people ask then you need to have something to hand to give them. And writing your website or your number on a used napkin is no good. Even if it is pretty. DON'T steal your photographers clients. At least not in front of them.
9) Pay Attention
It sounds simple, but most forget about it on the day! Take note of everything that your Photographer does. You're there to assist them, but also to learn from them. Take note of what works and what doesn't, do it silently. Don't write it all down in a little black book, that's too obvious. But pay attention and learn.
10) The Photographer is always right...
Need I say more?