In time you'll get to know your limits, but pace yourself, and stop drinking if you start to feel nauseous. Personally I can take four double vodka + mixers on an empty stomach and still be okay. Give me a fifth one though and I'll be sick.
DO NOT drink too quickly, otherwise one minute you'll think "oh I'm fine" and five minutes later you're absolutely gone.
Do not mix your drinks either - i.e. don't have beer for one drink, and vodka for the next. Terrible idea.
The other thing, if you drink on an empty stomach your tolerance is going to be much lower than if you've just eaten a substantial meal before going out.
Finally, drink a lot of water before you go to bed.