Experience gain in RTW is a little odd, maybe. Unlike RPGs, where you usually get more experience for killing tougher opponents, especially level bosses, in RTW a man is a man. Your guys get as much experience for killing a peasant as a faction leader.
I've been metagaming this some recently by having some units, especially younger faction members, key on the weakest units in opponents' armies. Partly as a result, I now have a 25 year old Carthagian faction heir with 3 silver chevrons. (He has several living 50-something uncles, including the faction leader.) This guy is by far the youngest faction heir I've ever had.
It seems to me that you have a slightly better chance to hold (or at least not degrade so much) a more experienced units' chevrons if you merge less experienced men (including retraining) into the more experienced unit than the other way around.The basic answer to your question is: KILL MORE PEOPLE! Only quantity, not quality, of opponents counts.
Merging units:
Whenever units are in the "same place", that is they show up in the "cards" in the lower middle of your screen, they can be merged by dragging & dropping one unit onto another EXACTLY THE SAME KIND unit. The "same place" can be in a city, a field army, or as passengers on the same ship. (Note that when an army is on a ship at sea, the "Army" tab is lit up - if you left-click on the Army tab, the land units display instead of the ship(s). If you have a really mixed passenger list of an army, diplomat(s), spies, & assasins on board, take care to not let the game offload automatically. It can (& usually will) really screw you over. Move the ship very near land where you want to offload. Click on the agents tab first, usually. Have them move off to spy, kill, sabotage, whatever. Then, with your "eyes" out in front of you, move the army off.)
[This message has been edited by mikecz (edited 07-07-2006 @ 08:56 PM).]