DS2 is 4 days old and my milk came in yesterday. He's feeding ok (latch could be better, but it's improving) but my boobs are MASSIVE and tender and lumpy. There are no hot patches or anything though...yet...
It's also making it even harder to latch him on properly as they're so full and round.
Should I pump some off to prevent blockages? Or will I just make it worse by increasing supply even more?
Try not to pump. Hand express a little if you're really uncomfy, and try hot flannels or a hot shower first.
It will get better, just keep feeding frequently to relieve the engorgement. Taking ibuprofen can help if it's really sore.
Congestilations too, of course! 
Thanks ohbugrit. Am waking up DS2 to get him to try to feed again rather than pump. Will try the hot flannel too
Ow ow ow
If you are very full/flattened you could try reverse pressure softening (it's a way to 'push' back the excess fluid you get with engorgement). Details here and video here.
Truth, in a previous incarnation of mine you saved my butt with that link!