When printing, make sure that you print onto the film so that the ink will be the side that goes against the copper, otherwise light can get underneath during exposure & your tracks become thinner.
Make some big "+" marks outside of the board edge on both films, so that when you have your board in between you have something to check the alignment with.
Make sure that your tracking/copper is well balanced so that you do not have loads of groundplane on one side and few little tracks on the other, otherwise you may find the thin tracks being etched away.