If you can't stream at some BPM, then practicing triples and quadruples at that BPM is a good way to ease into it.
Here's a good practice map I've used a lot. If you have problems streaming at 150 BPM, play the first difficulty. If you have problems at lower BPM, try playing the other difficulties on halftime.
But practice maps can be really boring to play, so I've just used this map as a start when learning some BPM. After playing it a couple of times, I switched to practicing on normal maps, because that's more fun. : )