Honestly challenges with a guaranteed prize motivate me the most. A couple sites (Digitals & SOTB, may be others as well) have all their monthly challenge gifts coordinate into a big kit, which motivates you to do more of their challenges. I've done all the SOTB ones this month but am struggling with a couple of the ones at Digitals, since they are so specific, ie you must use these pieces I give you in a LO. And while the pieces are nice, they really aren't my style or I don't have much in my picture stash to go with them.
I really struggled with one there with a military theme and you had to use one of 3 word arts. Well, I have zero military people in my life. I think I've had a couple much older family members in the military, but they are relatives I barely know and I don't have any photos of them. I did the flag lowering ceremony at MK LO for that one.. a bit of a stretch, but patriotic at least!
DSP has a few challenges too, most have a random prize, so only a couple people that participate will get a prize. I will probably do those, since I have so many DSP products.. I've been lucky on their challenges a few times.
SBE has a few challenges too, and I did one, and I will probably try to do more, but the month is running out. They have posting rewards for all too.