I've had good luck with Castelli and LG leg warmers. It helps if they have silicone grippers on both the inside and outside so they grip your leg and the inside of the shorts. And I've had some (ChampSys and Nashbar) which were absolute crap and never stayed up.

When it gets below 55*, I wear technical (breathable) athletic pants over my bike shorts. I have a couple different thicknesses of pants, depending on temperature and whether there is dampness involved. For actual winter conditions (30-40*) I have flocked lined, shell pants over the chamois shorts. You can get neoprene toe covers for your shoes that will keep your feet much warmer. There are also neoprene covers for the whole shoe for winter. A breathable base layer shirt, then a lightweight long sleeve jersey with zippered front and then a jacke with venting and pit zips. I have a heavier goretex jacket for winter, also with long pit zips. Venting body heat and moisture is very important in cold conditions too! A couple different weights of gloves. A helmet liner...one just covers my ears, the other my whole head. As someone already noted, start your ride just a bit chilly. yOu will warm up and be unzipping vents within 20 minutes.