Thai Plumb or clinch position: Both hands are on the crown of your opponents head (NOT neck), your elbows are squeezed together on teh sides of his neck with teh points of the elbows digging into the chest just below collar bone. For full control, pull your hands down adn put your chin on your hands. From this position, you are able to control your opponent and throw knees and elbows easily.
Here is a pic of a fighter attempting to get clinch control
BTW, this site has some excellent material and is well worth checking out.
Control: Once you get their head down, you don't just stand there, you whip them back and forth from side to side. The result if done properly is that your opponent is whipped around and is totally defensive.
Underhooks will be more advantageous for a shorter fighter, take a look at Randy Coutore fights at sherdogs.... bottom row, second from right is a decent picture of underhook control,http://www.sherdog.com/news/picture_..._id=&my_page=3
and here, 4th row down, first picture is Kevin Randleman (black guy) trying to get underhooks on Ron Waterman, Waterman is defending, randleman is attempting to link his hands together on Watermans upper back (between shoulder blades). http://www.sherdog.com/news/picture_...in%20Randleman
btw, teh entry type I described in my earlier post will give you head outside control, which works great for throwing knees to both the groin and abs, as well as the spine and tail bone. It's also a good position to throw and trip from.
hope this helps