My boyfriend used goo inside the tire of his RM 125, and it worked terrific. In fact, he did that at the end of last season, and it is still holding up - we have ridden 3 times so far this season, and he hasn't had to fill up with air yet. I have heard mixed reviews about it, but it definitely worked for him.
slime. Depending on where the hole is nothing will work but a new tube. I usually patch the hole then put in the sealant. Its a good idea when installling a new tube to just dump some slime(or whatever you use) in the new tube. I know a few friends that do this. I haver had slime in my back tube for 5 rides now.