What's a good way to strip paint off a bicycle frame to be painted?

Use a fine, circular, wire brush that attaches to an electric drill. I stripped a bike years ago doing this and it worked well.

If you have any kind of budget for the project I'd recommend sandblasting before you repaint... it's the quickest and most thorough. You can try calling auto body shops to check their rates... shouldn't be more than $30-$50.

Also, make sure you're *really* thorough when you repaint -- exposed steel is just waiting to rust as soon as any moisture gets in there.

wet sanding with sandpaper

Airplane thinner available at automotive paint stores. Just do it in a well ventilated place.

You don’t actually have to strip the old paint off before you repaint your bike. You just need to rough up the old paint surface sufficiently for the new paint to adhere. This can be done with sandpaper or as mentioned earlier, by using a wire brush with an electric drill.