Also make liberal use of the cropping, flattening layers, edit>purge>all, etc.
When you place something half off your document bounds, or you scale or free transform something so it's 5 times the size of your document area, the extra stuff is still there, you can drag it back into view with the move tool etc.
Selecting all, which will make a marquee as big as your document bounds, and then cropping the image, won't have any visible effect on your composition but it may reduce the size of the thing in memory dramatically as it will chuck away all those invisible memory-eating pixels outside the edges of your document.
They'll *still* be thrashing your memory usage by virtue of being kept around in the history so you can have a squillion levels of undo. Flushing the history, purging the clipboard, etc, regularly is a must when working on a big file to stop it becoming unwieldy.
Also, make liberal use of stuff like vector and type layers and layer effects. If you want a bit of solid type on your marquee with a bevelled edge and a dropshadow, say, it is a lot less memory intensive to define that as a type layer (so vector graphics defined mathematically by a few points rather than by thousands of pixels) and have layer effects set up to render a bevel and a dropshadow on the fly than if you rasterized your type, set up a separate layer with a blurred copy of it as a dropshadow, etc, since every one of those raster layers is adding a bunch more pixels to your document.
Also, consider that 300dpi may well be overkill. For litho printing, a common rule of thumb is to use a dpi of 1 and a half to two times the linescreen in lpi that it'll be printed at and things like coffee table books are printed at 150lpi, which is where the 300dpi thing comes from. Magazines are often 133lpi. Now, you probably won't get stuff printed using traditional screening, but rather output on a big inkjet which effectively dithers everything, and is a completely different concept. Display graphics on Lambda printers and such are often done at a lot lower res and they still look good. Plus you have to remember that the filesize you're working with will increase with the square of the dpi since you're working in 2 dimensions - double the resolution, you quadruple the number of pixels. Even a small reduction in resolution (working at 250 dpi say) may have a big impact on the number of pixels you're pushing around.