So I decided to join and get serious. What are your best advices?
Advice? Never stop. Maintaining is just as hard as losing and if you stop the discipline you'll regain. We have a thread in here Maintainers Facing Fall Fearlessly--it's where those of us who did stop the discipline gained some and are trying to relose the weight.
Thoughts on the birthday party? Smile and say no when the food is pushed your way. You can do it! You've come this far and you don't need your pesky coworkers to ruin it for you. But if you do give in, don't punish yourself. We're all human, we make mistakes. Just get right back in there and reverse the damage.