I like to warm up with a couple of easier jack and stream maps.
This mapset, for example, is good for a warm up since it contains jack, stream, and pattern mixup diffs.
After that I play a couple of maps with DT like
this one just to get my hands up to speed.
Usually takes about half an hour and then I can proceed to playing harder stuff and push the boundaries.
The maps I suggested might be too hard as a warm up for your current skill level, so just find maps with varied patterns that you can 950k+ and use those as your warm up.
Also, contrary to what Sasami-san said, starting with stamina maps does not work for me at all, so to each their own, I guess.