This is the quickest and safest method I have found for soaking stamps which have been been gummed to an envelope (meaning they are not self-adhesive or "peel and stick" varieties, which I always soak separately). First, you should not do above one hundred or so stamps at a time. You can of course do as many as you like in a bathtub, but it gets harder to do a really good job, because the water becomes rather dirty from all the excess gum. I do each lot in a washbasin or sink as follows:

1) I use fairly warm water, but not hot;

2) I put the stamps in enough water to be able to swirl them around a bit, so they don't remain lumped together - this ensures they soak individually and shortens the time you need to wait;

3) I can usually start to pick out stamps after only a few minutes - some will unstick almost at once, in fact, but it is advisable to wait 5 minutes so you have a fair number that are ready to come unstuck (or have already done so, by themselves);

4) I start by removing as many pieces of paper (not stamps) as I can. This allows any free stamps to float around a bit and lose as much gum as possible;

5) I then gather up stamp after stamp, peeling them off their backing paper when necessary, and leave them “stuck,” as it were, to the side of the sink, one on top of another, above the waterline. I can do a whole sinkful of 100 or more in about 5-7 minutes. Once all the stamps are out of the water ("stuck" together in groups if they represent different sets or types, or in any order if they are all more or less the same) I pull the plug and if I want to go extra quickly, I rinse the sink a bit and start another lot;

6) I then carefully take one or more batches of stamps from the side of the sink and then peel them off one by one (they will usually slide apart without much trouble) and lay them carefully in rows on newspaper to dry;

7) For an extra good job, you should either shift them slightly on the paper after about half an hour of drying time, or move them to a fresh sheet of paper entirely (but this is obviously somewhat time-consuming). This ensures that they don't actually stick to the newspaper.

I use this method because it is fast and efficient. There is not very much water transferred to the newspaper so the stamps dry faster and tend not to stick as much. Of course, you have to move a bit quickly anyway; you cannot leave the stamps to sit in piles on the side of a sink for too long. If you do get interrupted, you can of course simply put them back in the water for a minute and continue the process. One last thing: it does take a bit of practise to do this, but not much. However, you should not start with any stamps that you consider to be of particular value. Try it out first with common stamps that you won't mind damaging.