IWB, aka: Mexican carry.
A brown or black shoelace (depending upon trouser color), with a loop tied in each end for the belt. (I use leather boot lacing, but cloth laces work just as well)
The straight length should be adjusted for a particular gun, so the when it's tucked over the top of the beltline into the waistband, it is long enough to hold the gun below the beltline, excepting the grip.
When "holstering", insert the muzzle between the straight section and the inside of the trouser beltline.
The retention point for revolvers is the cylinder bulge, autoloaders is the front of the TG.
The trouser belt should be tight enough to retain the gun from moving upward from walking/running, etc - but not tight enough to retard the draw (practice).
A good commercial option I've found for warm weather wear is Thunderwear:
http://www.thunderwearholsters.com/
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