Tempering chocolate without a thermometer
Your best bet is to start with a good quality tempered chocolate and melt it just to the point of melting, which is to say keep it 88° - 91°F (if you ever do end up getting a thermometer). Most suggest using a microwave for this in short bursts to keep the chocolate from getting too hot.  If you use a double boiler, use a low heat and go slow.  Pull the bowl off the heat often and stir.  It may not seem like all the chocolate is going to melt, but keep stirring and add small amounts of heat.  Or you can follow Alton Brown and sandwich a heating pad between two bowls and adjust the heat.  

The theory behind this is there are 4 different types of crystals in chocolate.  The good ones we want are Beta crystals, which do not melt until 91° - 94°F.  As long as you don't  melt the beta crystals, the chocolate should set back up to a proper temper.

Use a grater to make nice even bits, melt the chocolate 2/3 of the way and then remove from the heat.  Stir off the heat until the rest melts, or add heat in very short bits, 5-10 seconds at a time.  Don't let it harden either, it's harder to remelt if it's hardens up slighty as it then becomes one big solid mass to melt.  

Once you break the temper, there is no going back, you have to start over and there is no real easy way to do that without years of experience as bobobobo said, or a thermometer.  You can find inexpensive thermometers online, just have to make sure to get one that handles the lower range of temperatures.  I use my chocolate thermometer to also measure the temperature of water when making bread.  It seems more accurate at the lower temeratures.