Tip 1: Don't be a jackass
You are still at work, and you are still representing your company, rude remarks and vulgar jokes are out of the question. Act professionally like you would do at work in the company of those managers.
Tip 2: Don't be a jerk to the staff
Sure maybe the waitress and the cooks screwed up and put your pickles in at right angles when you specifically asked for them to be parallel, but don't be an ass about it, yelling about it and causing a commotion is going to look bad on you, even worse, it will look bad on your company.
Tip 3: Avoid alcohol hic
When ordering the drinks try to avoid alcohol, remember, still a work environment. The potential exception is if the managers also all do it, but even in that scenario, you know your limits, I hope, don't over do it, stick to one if you can. You still have to go back to work afterwards
Note: Be wary of this, this is different in different cultures. If you are unsure of what is acceptable then play it safe and get a soft drink
Tip 4: Red Lobster
I don't know if the company is paying or if you are all paying for yourself. If its the former, then don't order something pricey just because you can. Order something that you would normally order if you were the one paying for it. You never know, you just might be, and you don't want to be stuck with a bill that you can't pay.
Tip 5: Musical chairs, Where to sit
This I can't really see as mattering much, it's just sitting for lunch. Just don't plonk yourself in a seat and refuse to move if two other people want to sit next to eachother, given you don't really mind where you sit its easier to just avoid confrontation.
Everything you should be doing should paint a picture of you as a professional, smart employee who doesn't snap at the slightest thing. This is especially important in the company of your higher up's, acting a fool would be a bad career move.
Tip 6: Loosen up, but not so lose that your pants fall off
Don't actually loosen your trousers, this was metaphorical.
Just relax and enjoy the lunch and probably reminisce about the employee who is leaving. Don't panic too much about the others there, they are just people too.
Tip 7: Don't make it all about you, that's MY advice to you
Sure you are now in the company of many higher ups and have the perfect opportunity to show off about what you've done to hopefully gain some attention. But don't. This lunch isn't about you, its about the employee who is leaving, keep that in mind.
Tip 8: If you like it, put a ring on it. Engaging at lunch
Don't think of them as 'Your team' and 'managers'. You are all employees working for the same company. Engage with everyone where appropriate, don't butt into conversations, but don't sit there silently and ignore everyone either.