My worthless opinion (or maybe it is worth USD 0.02).
You should try hard not to get up when the seat belt sign is on.
You should try hard not to get up during meal service time.
Do not wait too long before getting up to use the lav when needed; the seat belt sign might come back on and for a long time.
You should not wait until the end of the movie to use the lav.
Try to do everything you need yourself, then ask for help including from the flight attendant if needed.
If you are in the middle section of a 2 aisle airplane and there are an equal number of people on each side, do not always get out on the same side.
More worthless info:
http://www.cockam.com/travel.htm