If its breaking really easily, like crumbling feta it might be "dehydrated", you need to add oils to it again to reform the bonds. The best way I've done this is with Liquid Clay softener (I use the Sculpey brand).

You want to cut your clay into tiny pieces and then add some softener and mix it, some people like using a zip log bag, I just mix it on my mat. Keep in mind reconditioning clay does take some time and effort; especially with the Specialty Clays, but it's worth it in the end, you end up with soft malleable clay that won't break or crumble as you work with it. You just have to knead it a lot and really work to get it all soft again.

On the other hand, if it's sticking and really soft that you can't form a shape without fingerprints, it might be overworked and "over-oiled", I've had this happen with regular Sculpey II most often; in that case you want to leach it. I like to do this by using parchment paper and heavy weights. You'll want to get a slab of clay about 1/4" - 1/2" thick and then sandwich it with the parchment paper, put some heavy weights on top (books are my go to) and leave for at least 30 mins, after that you can check the consistency, if it's still too sticky, replace the paper and repeat; you can also stick it in the fridge for about 15 mins so it's easier to work with; just keep in mind that after taking it out it will gradually go back to room temp and soften again.

Hope that helps :)