At least Ryanair uses both front and rear doors for boarding and dis-embarking, and the best advise I can give you is to chose the rear
door and take a seat in the back of the aircraft: Most passengers (such as BY188B
) take the front door, so the chances to have the middle seat free are much higher when you're in the back. I've been on several Ryanair flight with load factor about 90%, all the free seats were middle seats in the back.
Regarding exit rows: As BY188B said, it's free seating, so you should be among the first passengers to board the aircraft. Boarding is done by security (sequence) numbers, on Ryanair "priority" boarding is number 1 through 65, on Easyjet it's 1 through 30 think.