A lot of people have a hard time deciding what pieces of equipment they should work on upgrading, especially when getting into PvP, where you basically just shoot for the best you can get.
My solution was to make a chart (using Google Docs) of all the gear I had my eye on (in this case, I limited it to PvP gear) and entered the Item Level next to it. Note that Item Level is not the same as Required Level. You can find an item's Item Level easily in WoWhead or Thottbot or whatever you prefer.
Next, I entered the corresponding Item Levels of the gear I currently have. Then, in the next column, I have it auto-calculate the difference between what I have and what I want.
Once that's done, you can easily see how different the items you WANT and the items you HAVE are, and can basically just go down the list, from most-different to least. For me (in the chart above) you can see that I'll be replacing my Bracers next. Which is actually pretty obvious if you've ever seen my bracers.