There are some static rewards that specifically address UMD. Generally though.
At low levels you can get a command item, that adds +2.
Later you can get the golden catouche from deleras, that adds +3. Does not stack with a command item.
Get CHA items. Each even number you get it to above 10 adds +1 to UMD.
You can buy pots/gather collectibles for pots of something or other that add an alchemical bonus of anywhere from +1 to +3 to CHA based skills. This will stack with any other bonus. You can also get an alchemical bonus to CHA which will bump the UMD further.
You can get heroism or greater heroism. This adds a morale bonus to UMD, which will stack with the three above.
Last but not least, you can get a luck item that will stack with everything above. +2 from the reaver is the highest I think.
Also, on items you need to see what king of bonus it is. Command and cartouche give a competence bonus. I am not sure if others give an enhancement bonus. If they do they will stack with command/cartouche.
To my knowledge no bonuses to skills will stack, so you need to get one of each kind and layer them to get the highest possible.
Edited: there is also the green steel items. But I am not sure what kind of bonus they add.
Stack rewards is herre.
http://forums.ddo.com/showthread.php?t=121279