I accidentally got lube on my rear brake (v-brake) and my stopping power seems diminished. What is the best way to get it off the brake pads and the rims?
Clean them. I'd avoid using any sort of solvent, as it would be bad for the pads and might simply make things worse.
First undo the noodle so you can access the face of the pads, then wipe pads and rims thoroughly. Scrub pads and rims with a dab of detergent on a rag, then hose down to remove the detergent. Reattach noodle. (I always forget that part.)
The brakes will likely still be a bit "slippery", but riding with the brakes partially engaged for a minute or two should bring things back to normal.
Loosen the V-brakes, so you can access the pads (you can even remove them and clean each individually).
Then apply degreaser to them (there are many types, I end up by using a citrus-type or in some more radical cases even use petroleum on the rim, but make sure you clean it with a rug afterwards!).
NOTE: using a solvent or petroleum can damage some parts of your bike, so make sure you test it first and definitely don't use it on rubber!
Do the same to the rim.
Clean it very well with a rug and check if there is any trace of grease, if it has, try to degrease it further.
When everything is ok, just reassemble the brakes and test it, if you did everything correctly then it should work ok!
My recommendation is to use a rag and de-greaser like Simple Green. Apply the solution to your rag, wet pads let the liquid do its thing and wipe clean/dry. If that does not work, repeat. If after doing that twice you are not getting the appropriate braking power, I would gently sand the face of the brake pad with a fine grit sandpaper.
V-brake pads are rubber. Degreaser can (but not always does) damage rubber. use isopropyl alcohol. It will remove the grease without damaging the rubber pads. It may take multiple applications. Clean the rims with it as well.
Quoting from Daniel's answer, with edits:
First undo the noodle so you can access the face of the pads, then wipe pads and rims thoroughly. Scrub pads and rims with a isopropyl alcohol on a rag. It will evaporate.
Repeat as required. If more is required, sand the face of the brake pad with a fine grit sandpaper.
Reattach noodle. (Everybody always forgets that part.)
Hope that helps.
I'd use Simple Green or a similar mild degreaser followed by denatured alcohol to eliminate any residue. Make sure and thoroughly rinse Simple Green off your rims and pads using water, it makes the brakes grabby, loud, and yet ineffective if you leave it on.
You could also lightly sand the brake pads to get rid of the contamination.