The rules of tech trading are simple:
If they're researching something that you don't want them to have (say Flight, to cancel your Colossus

) then you should bung as much junk tech their way as possible to slow down their research.
Never give them anything that may lead you to directly lose a wonder or a city. If they are building a wonder and you're nearly finished Mike's then you just know that giving them Monotheism will mean they'll change wonders and beat you to it!

If your cities are not ready for it then avoid giving the AI military techs. No city walls? Then don't give away Mobile Warfare or Monotheism, even if you've done Mike's. Don't give away Advanced Flight until you've got SAM batteries in all cities in range.
Try gifting to your key civ. This will be the purple civ if your power rating is supreme, so get the Mongols (or whoever) down to a single city, ensure that they can't grow much or gift on the techs you give them, and then give them everything! This is best at the end of the game, as they'll never have the chance to build a spaceship with a size 2 city and one shield. Have a look at the first Civ 2 Demo game in the archives for a lesson in how to, and how not to, gift in this situation. Bloody Mongols gave Advanced Flight to all the other civs!
Also remember that swapping techs will cause your own science rate to slow down so be absolutely certain that you want what they're offering! If you reckon that you can beat them to a key wonder (they are unlikely to give the techs away though) then go ahead, but if this new found knowledge will mean you miss the Oedo year for Monarchy then you'll be kicking yourself.
