any tips on how to "freshen up" my 05 YZ125?

Personally I would pull it completely apart and grease every thing that can and should be greased. Not only will you find out what really needs to be replaced, but the odds of having a problem with the bike due to improper maintenance goes way down.

Wheel bearings can be greased by popping the seal out. Pull the swingarm, linkage, chain adjustor bolts, triple clamp and grease all of them. Clean the cooling system by running it for a few minutes with 1/2 cup of white vinegar added to the water, then thoroughly flush the system before using your favorite aluminum friendly anti-freeze/water mixture. Repack the silencer, no need for a new one unless yours is broken. The shock and forks should be serviced too.

Armed with a factory manual, some time watching videos on YouTube and a few specialty tools you can do most if not all the work yourself. Take lots of pictures while disassembling stuff, it helps cut down on having parts left over when you're done with the reassembly process.

Check for the dreaded shift stopper issue. Present in 05-06 YZ's.

http://www.thumpertalk.com/topic/947435-is-a-yz-125-enough/page-2
If it's a bushing on your bike it's okay. If it's a bearing it needs to be replaced.

Check out this thread http://www.vitalmx.com/forums/Moto-Related,20/Yz-125-Issues,1198546

http://www.thumpertalk.com/topic/512858-replace-your-shift-stopper/

Buy the service manual and I'd change the shift stopper before even riding it. Everything can be done from the clutch side (those links to thumper talk are great..)The stopper is little bit of a hassle as the spring is under tension but be patient it's not too complex.

It doesn't matter how hard you ride it, if that shift stopper comes apart you might need more than just a topend replacement. As soon as I heard about this issue I stopped riding and replaced that part....... I wouldn't think twice about...if it was my bike!