Anne Mini recently did a great post on why it's a good idea to have two spaces after each period. This was actually not a habit I ever picked up, as I don't believe my typing teachers ever advocated this (my wife learned this in her schools, but I didn't -- maybe I just missed the boat). At any rate, when I heard of this preference a few months ago, I decided to make the shift to using the two spaces, and in order to do that I had to reformat my entire WIP manuscript.
Hours of tedious work? Not hardly! Using nothing more sophisticated than the "Find and Replace" function in MS Word, here's how to reformat your manuscript to add two spaces instead of one. Just follow these steps in order, and you should be good to go. It also works on manuscripts that are half and half -- part of it formatted with the correct two spaces, part of it formatted with only one.
Of course, as with any large-scale change you make to your manuscript, make sure and save a copy first! If something should go awry for whatever reason, you don't want it to happen to your only recent copy.
Now, on to the steps:
1.) Open "Find and Replace" in MS Word by hitting Ctrl+H in windows, or by looking under the Edit menu and choosing "Replace" (the menu location may vary slightly by Office version, especially in the 2007 edition, which moved the location of a lot of things -- I'm using Office 2003).
1.a.) Step two and below will use the character "_" to represent a literal space (since you otherwise couldn't see it), and square brackets  to indicate the boundaries of the text you should be finding and replacing. When entering these values into your textboxes, omit the square brackets, and use a literal space where I show the underscore.
2.) Search for [._] and replace it with the value [.__]
3.) Search for [."_] and replace it with the value [."__]
4.) Search for [?_] and replace it with the value [?__]
5.) Search for [?"_] and replace it with the value [?"__]
6.) Search for [!_] and replace it with the value [!__]
7.) Search for [!"_] and replace it with the value [!"__]
8.) Search for [:_] and replace it with the value [:__]
9.) Search for [._._.] and replace it with the value [...]
10.) Search for [...__] and replace it with the value [..._]
11.) Search for [___] (three spaces) and replace it with the value [__] (two spaces)
Unless you have strange formatting or other interesting punctuation, that's it! And I'm sure that you can see how you can apply these general principles to other types of punctuation, or strange types of speech-delineation, like the carrots (<>) that are used to identify thought-speech in Orson Scott Card's SPEAKER FOR THE DEAD.
The key is to run all your replaces that are like 2-8 first, so that you have at least the correct number of spaces after each sentence; then run numbers 9-11 to collapse any inappropriate extra spaces back down. If you're worried that things have gotten very out of whack, you can even run numbers 10 and 11 repeatedly, until they find nothing more to replace (that's actually not a bad idea to do, anyway, just to be sure).
I used this logic to successfully reformat my own manuscript several months ago, and I haven't found any problems with it in the interim. But I'm not the biggest expert on the rules of two spaces versus one, so if you see a hole in my logic, or if there's something I'm omitting, please let me know and I'll update the post. Happy formatting!
UPDATE 1: Someone pointed out on Anne's blog that this sort of logic incorrectly adds too many spaces after abbreviations. Since I didn't use any of those in my WIP, I didn't have that problem. But adding lines to search and replace things like [Mr.__] with [Mr._] would work, presuming you know, or can at least guess, all the abbreviations you used.
UPDATE 2: The list has been simplified a bit regarding quotes, thanks to a comment to this post.
UPDATE 3: Actually, that simplification was invalid, as another comment pointed out. In order to avoid adding extra spaces in cases like [what I think,"_said Tom.], the full steps above are necessary.