Bottom heat, but remember if you force things to grow quickly, they may not develop into their true shape and will need protection if grown in heat.

You need to try and maintain optimum conditions for each individual species. Usually this means keeping them constantly warm (usually between 15 and 25 degrees C, depending on what you're growing) and humid by sealing within a propagator or clear plastic bag. Read the instructions on the packets carefully as each plant requires slightly different growing conditions - one size does not fit all unfortunately.