It's worth mentioning that the common AK-47 does not have an automatic bolt hold open mechanism. New AK's may appear to have a manually operated one due to the tendency for the bolt to stick to the rear when manually cycled, but this will stop working with use over time as the surfaces polish each other during normal operation.
A commonly used "poor man's bolt catch" can be made by using a dremel or roto-zip to cut a notch towards the rear of the safety. This leaves sharp edges on the AK, which can snag or cut things.
In order to lock the bolt open with this mechanism, you would pull the charging handle back, then flip the safety up so that the notch you've cut captures the charging handle. Be careful not to cut the notch too far back, or the charging handle won't be able to reach it. Also, make sure you cut it at the right angle. If cut at the wrong angle, the charging handle will have a "ramp" to ride forward, pushing down the safety along the way and closing the bolt without warning.
Bolt Hold Open Magazines
Yugoslavian magazines, as well as a number of domestically produced modifications, will hold open the bolt of an AK with the last round fired. However, all the variations of this I've seen will only hold the bolt open while the magazine is in the rifle - as soon as you release the magazine, the bolt closes, unlike most rifles (like the AR-15) which require you to hit some kind of release before the bolt will close again. This means that every time you change the mag on an AK with an empty chamber, you must manually cycle the charging handle again.
Variants with Bolt Hold Opens
Some, but not all, Saiga rifles have a manual bolt hold open which is operated via a small button placed a bit forward and to the right of the trigger.
-- SeanNewton - 22 Apr 2009