Warm water works if it's not super-cold outside. You can pour warm water over your windshield and use the wipers to clear it off. If it's really cold outside, all you are going to accomplish is adding a thick layer of ice to your windshield (best case) or cracking your windshield from the temperature difference (worst case).
Saltwater works for the same reason salt works as a de-icing agent (and warm saltwater would be your best bet). The ions in the salt lower the freezing point of water, so a little ice will be melted by the saltwater. This water will attempt to re-freeze, but the temperature will need to be colder than 32°F for that to occur. All salts aren't created equal for de-icing. Normal table salt works when it isn't really cold. Road salt, which has a different chemical composition, works better when the temperature is very low. Salt exposure isn't great for your car, so keep that in mind. Actually any chemical that you add to water acts as impurity and lowers the freezing point of water, so wiper fluid would melt the ice more quickly than pure water, too.
A quicker way to melt the ice on your windshield is you already know... placing your bare hand against the windshield. This works because (a) your hand is warm and (b) your hand is solid. Warm solids contain more particles per unit area to convey heat to the windshield than you would get from warm air (molecules in air are very far apart). So, any warm solid will de-ice the windshield better than air. Any warm liquid will de-ice better than air for the same reason (ice melts faster in water than in does in air of the same temperature).
You don't have to use your hand. The base of a warm shoe would work; so would a warm book. The more dense the object is, the more effective it will be. Therefore, you'll get more de-icing power from your shoe than you would from.. say... your sock. The heat capacity of the material matters too, which is part of why your hand is such a great defrosting tool.
If it's not too cold, I'd probably go with a warm, damp towel on the windshield. If it's bitterly cold, the ice scraper is still your best bet. If you have a garage and live in a warm climate, you can prevent the frost from forming in the first place by parking inside.
What are your best tips for de-icing a windshield?