Perlite & vermiculite
Perlite and vermiculite, combined at a 1:1 ratio, make an effective, cheap, and widely available cloning medium. Simply mix 50% perlite with 50% vermiculite, add enough water so it's moist, and fill your small containers.
The plants to be cloned should be on a vegetative light cycle (18 to 24 hours of light) and should not be overfed. Excess Nitrogen in the stems can slow down rooting. You can take cuttings from plants that are blooming and heavily fertilized, but your success rate would be lower.
1) Prepare your mother plants at least 6 hours in advance with a fresh drink of water so they are fully loaded before you cut.
2) Using a screwdriver, toothpick, or alien probe, make a small hole in the medium, about 3 cm deep at each clone site.
3) Take the growing tips from the mother you wish to propagate with sharp scissors-- they should be around 6 to 9 cm in length.
4) Cut off all leaves except for the top 1 or 2 plus the small growing shoot.
5) Recut the stem end with a sharp X-acto or scalpel, making it as angled as possible for large surface area contact and water uptake.
6) Optionally dip the cut end into a rooting hormone containing fungicide (Rootone F is cheap and widely available, very effective)
7) Insert into the medium, gently pat the hole closed. Keep under fluorescent lighting, misting them daily for the first few days if your climate requires. You might have to lightly water the medium once or twice before roots show in 7 to 14 days.
There are different "grades" or sizes of perlite and vermiculite. For cloning, I prefer the small to medium perlite and the medium to large vermiculite, but don't worry if you can't find the exact size you're looking for. The 50/50 mix holds water rather well when there are no plant roots sucking it up, so don't overwater your cuts.
For an alternative view, read Oldtimer1's excellent mother/clone series. You can easily substitute 50/50 P/V into his system of taking cuts.