What to use to top off after racking?

I'm going to rack soon and what can I use to replace the lost volume? Boil some tap water? How much headspace to leave? Wish I would have saved a gallon for topping off!

Most people start with a larger volume and anticipate the volume lost during racking and from the trub settling out. For a 5 gallon batch (ending with 5 gallons): Start with about 6.5 gallon boil, figure about a gallon lost during the boil = 5.5 gallons into the fermenter. 0.5 gallons lost to transferring and trub = 5 gallons in your keg or bottles.
I do not recommend topping off after racking, its not worth risking the batch just to get a half gallon more. Plus you are going to water your beer down and your starting gravity and final gravity wont really mean anything because and water will change your final gravity (lower it). But if you must, just boil some tap water, cool it, put it in to the secondary and then rack on to it.

EDIT: Just realized this is in the cider forum...., I still wouldn't top off though.

If you're going to top off, use 100% apple juice or cider...

I'm pondering this myself. My two one-gallon batches, already in secondary, have a lot of sediment and I'm considering racking them again (tertiary?). Problem is, one's already almost a pint down, the other a little less than that. If I rack again, how much should I worry about oxidation?

My first batch of cider I added glass marbles to reduce the head space -- something I read in an old wine-making manual. Then later afterward I noticed the marbles were made in China, so now I'm all paranoid about lead. (Not a recommended additive to alcohol, as the Romans might tell you.)