Im doing planks after set of leg raises to kinda finish my abs. Im just holding it as long as I can (or a bit less). Also when Im doin plank I place my hands closer together, feet wider (shoulder width or less) and slowly touching side of my legs with one arm (alternating them of course) and trying to not move/twist my body at all to challenge my core to stabilize my body.
Anyway you can build time up to few minutes, then try some harder excercises.