Some additional info would help narrow things down but here goes.
I assume that the problem is something that slowly drains the battery (like overnight). If it were to drain it very quickly, the problem would be obvious (alternator winding smoking because of bad diode etc) or it would simply blow a fuse. I also assume the battery is ok. You could disconnect the battery, leave it sit overnight (or however long it takes to drain the battery) and see if it will start after reconnecting.
Frequently, a slow mysterious drain turns out to be the light in the trunk, glove box or under the hood that doesnt turn off when it should. If the light is controlled by a mercury switch, parking on a hill may keep it lit. Have you added anything to the vehicle like a big stereo amplifier? Maybe it isnt turning off.
If you disconnect the neg battery cable and connect an ammeter to complete the circuit again, the meter will show any current being drawn from the battery. In most modern vehicles there will be a small drain (less than 0.02 amps) to support the clock, radio station presets, engine computer etc). Anything much more than this indicates a problem. Start pulling fuses, one at a time to see if the current drops off, if so, you will have at least isolated the problem to a small part of the vehicle wiring and will have to narrow it down from there. Try disconnecting anything that had been added.
If the vehicle sits for a week or more, the small (normal) drain could be enough to drain an old battery and prevent starting. Do you have a cell-phone charger or something plugged into the cig lighter socket?
Depending on how much current is flowing, it is sometimes possible to detect current flowing thru a wire by holding a compass next to the wire to see if the needle deflects. Not a perfect system but it might work. There are "clamp-on" ammeters which would make it real easy to see which wire the current is flowing on but they are not cheap.