Know what you're painting over. If you can't find out what kind of finish you're covering, then you need to buy alkyd paint. It bonds with just about anything. Latex is easier, but it is finicky with some surfaces. It's better to be safe than sorry; so choose the alkyd if you're unsure.
Purchase latex if you have a lot of moisture and dampness around your home. Latex is porous. It allows moisture to escape and the surface to dry. The bonus to this is that because it has no vegetable oils, it also retards the growth of mildew, which is a problem wherever there's dampness.
Paint masonry with latex also because it usually doesn't require a primer. That will save you a great deal of cash as well as hard work. Paint when the masonry surfaces are slightly damp. The paint bonds best then.
Apply a rubber-based, solvent-thinned paint on cinder blocks. The water content in latex seeps into the blocks. This lets the iron particles the blocks are made with rust and stain your new paint job.
Paint over aluminum siding after it's well-prepped: sanded, washed really well and primed with a primer that contains a zinc base. Then you can use any paint you choose. Still, latex does offer the easiest cleanup.
Apply a glossy finish on doors and windows, which get handled constantly. Not only does it make the paint hold up a great deal longer, it gives the trim a nice sheen, as well.
Try a little bit of the color on a small sample to see the color when it's dry on your house. The materials you're painting over and the finishes can change the paint color quite a bit. Sometimes, the paint itself changes when it dries.
Test stains to be extra sure of the color, they are notorious for drying to a different color than you expected.