I switch it up. I usually almost always start with lower ab work (leg raises, flutter kicks and such) and sometimes do where I cant do anymore ... or sometimes I will do sets of 50 ... or sometimes I will do like a large super set ... doing all the ab exercises in like a circuit ... only doing like 25-30 each ... but continuously going from one excercise to the next ... a couple to three times.
I like keeping my muscles guessing, and keeping motivated
Some of the excerices I put into the mix are leg raises, flutter kicks, bicycles, v-ups, crunches, rocky crunches ( I call them ... lol), leg up crunches, side v's, pelvic/ab thrusts ... hmmm.... all I can think of right now.
Sometimes I will use a large rubber excercise ball in these, sometimes I wont.
Hope that answers your question.
Humbly,