I am assuming that the wire AND the connector terminal want to fall out togehter, and it is not the case of the wire wanting to pull out of the connector terminal; is that correct? If so, look at the connector terminal that wants to fall out; below the part where it mates into the other connector, there will be a little rectangular tab. That should be bent out at a slight angle, and is what locks the connector terminal into the connector body. You can bend it out a bit and it migth lock in.
Also, if the terminal that wants to fall out is a female circular conenction, bend it inward to make a tighter circle so that it will be tighter on the mating male connection. You also need to make sure that this is very clean then put some dielectric grease on it to prevent future corrosion. You can get this at any auto store; the local Autozone had them up at the counter in little ketchup packets.
If the temrinal is really corroded, or is pulling off of the wire, then IMO you are better off bypassing with new spade terminals on either side. Just use a good crimper or solder them; clean any soldered wires with alcohol afterwards to get the flux out, or it will tend to corrode the wires.