Don't go overboard - no need to take 4 months of toiletries. There is a PX with variable stock for the basics and it occasionally gets iPods etc.
Some of the accommodation (mainly the US stuff) has wireless internet access, so worthwhile taking a laptop if you know where you are going to be. Brit accommodation did not, but was planning to get it - again worth checking before you drag expensive kit out that will get manky in the dust (gets everywhere).
As for issued kit, depends on what you are going to be doing and where you might be going. If you are staying inside the wire most of the time, you can make do with the issued kit and some sports kit for the gym. Laundry is fairly good, but did get a couple of items trashed, so don't put in anything you care about - there are always self-service machines if you must.
The place is dirty all year round, either dusty when dry or very muddy when wet, but is approaching end of rainy period I think. To be honest, I would take as much issue kit as you can (difficult to exchange) and as little personal kit. Comfy boots a must, as everywhere is a decent walk. Screw top mug and microfibre sports towels (ebay) my best purchases.
There is a minimum kitting list (that includes S10 etc) on the HERRICK warning instruction, but ny specific questions, please ask.