I sometimes warm up with chromatics & scales, or pieces like Steve Vai's Solo from crossroads movie... Paganini's, Jason Becker , Malmsteen's or other fast stuff to get my hands warm & in shape.
If I'm learning a new riff, I just keep on doing it slow till I memorize the pattern, then I raise up my speed gradualy to the point I want... If it's a very hard riff, I put it on my practicing agenda for daily practice till I totaly tackle it.
Normaly bec of my studying, I just try to keep up daily practice by playing about half an hr, increased to 3 or 4 hrs in weekends (if I didnt' have extra studying) ... and on holiday I like to spend as much time as I can with the guitar, jam with bands and do some gigs.