Some of these guys are retarded. It can be hard to get in, especially if you are a bigger dude. It is all about kicking your feet like you are swimming, keeping kicking, pull hard on that handle, or the chicken line (which should be rigged very tight), preferably next to a D-ring. Flounder up onto the tube, lock those elbows like they said, keep kicking and pulling until your center of gravity is good enough that you can get a leg in, or grab onto something in the raft like a cam strap. I usually keep a cam strap around a thwart, even if I don't have anything to strap in, just for an extra place to grab onto if I need a little more pull.
I usually try to stay in the boat, or have someone in there that can give me a pull, but if you flip or everyone falls out, you will need to get back in by yourself.
I've heard of people using the prussic loop (google it) they carry in their PFD as a foot hold, and others who kind of use the chicken line as a stair if they can get a foot up.
It takes a big man to cry...It takes an even bigger man to laugh at that man.