First, decide whether you want your crawl space heated, warm, or cold. (search keyword "crawl space" for the title "BASEMENT: HEATED SPACE, WARMED SPACE, OR COLD SPACE?")
Insulating under the floor is usually done by first stapling a plastic vapour barrier to the underside of the floor. This is not very effective if the edges are not all caulked to the joists, as air leakage around the plastic will render it useless. Either do a good job or don't bother at all. If you have plywood floors, or vinyl floor coverings above -- you have a vapour barrier already.
Fiberglass batts are usually shoved up into this space. It is important that they be tight against the floor boards, and not hanging down. If the space is completely filled you can hold the batts up with a wire mesh or even "house wrap". Do not support it with a polyethylene sheet -- this is a vapour barrier on the wrong side of the insulation.
If the batts do not fill the whole space you will need to use "tiger Teeth" or spring-metal clips that bite into the joists and push the fiberglass up. RSI - 4.9 (R - 28) batts are thick enough friction fit to hold themselves up if the joists are properly spaced.
Insulating outside the perimeter can be done exactly as described for outside a basement. (search keyword "crawl space" for the title "BASEMENT: INTERIOR OR EXTERIOR INSULATION?") If there are no weeping tiles, only foam boards should be used, no fiber materials.
Insulating inside the perimeter of a crawl space can be done in the same way as inside a basement, with the following special notes:
-- Fiberglass batts should no touch the soil (although they could lie on a plastic sheet which covers the soil).
-- Rubble or stone foundations are usually quite shallow, and insulation must stop at no more than one foot below grade level or frost heaving is likely to occur.
-- If there is no danger of freeze shifting (search keyword "heat loss" for the title "BASEMENT: FULL HEIGHT OR PARTIAL HEIGHT INSULATION?"), additional heat savings can be gained by extending insulation horizontally along the ground (600mm, 2ft) in from the wall. This will have the same effect as if it was put vertically into the ground down to the footings.
-- A 6 mil polyethylene moisture barrier on the dirt floor will control humidity problems and raise the temperature of a crawl space by as much as 6 degrees C. (search keyword "moisture" for the title "WHY PUT A MOISTURE VARRIER UNDER A HOUSE?")