1 part vermiculite
1 part perlite
1 part sphagnum peat moss or coir
|Clockwise from the top: vermiculite, perlite, peat moss|
Coir is made from coconut husks and is more sustainable than the peat moss (due to the potential destruction of the bogs in harvesting the moss), so coir is preferable if you can find it in your area. Garden centers tend to have these materials readily available.
By adding 1 part compost to the formula above, you can make a nice potting soil mix as well.
If you plan on keeping your seedlings in their containers for a long period of time before transplanting, adding compost as a fertilizer is a good idea. You can also add compost to the original mix, but I feel its heaviness and water retention properties aren't ideal for beginning seedlings, which need air circulation to promote germination and are prone to mold if too water-logged.
Are you beginning to think about starting seeds, too?