I'm just getting back into lifting after 9-months off (unrelated injury). Where lifting (squats, primarily) helped me the most is with medium hills/rollers. After lifting a few months, I could do these segments with much less effort, leaving matches to burn for more serious climbs or sprints. What a trainer told me, and I've since read this about Peter Sagan's training, is that if you're gonna lift, go heavy/low reps on the lower body (working your way up and using good form, of course). Training maximum power supposedly gives you a more economical pedal stroke at threshold. If you're doing light weights and a lot of reps, you might as well hop on the bike and do hills, which would add a cardio benefit. Also -- both to prevent injury and encourage proper form on things like squats -- and also to improve pedaling efficiency -- work on hips and core stabilizer muscles in the gym. Lots of literature on the subject and I'm sure lots of people on here more qualified than I to explain.
One thing I have not quite figured out is how to balance lifting with riding and -- most importantly -- with recovery. If I lift on days I don't ride, do I need to add a no lifting, no riding recovery day? I can't imagine a lifting (lower body) day can count as a recovery day, but if I do two days of lower body a week, plus a rest day (or two), that really cuts into riding time.