Well, upgrades depends on your need. Something what you consider an upgrade might not be the same for a lot of others. You need to think a few things and get decided on the specs you'd like the new lens on and make a priority list of the features.

• What focal length are you planning on? Do you need a telephoto or a wide angle? If you shoot birds, you need something with a minimum 300mm on the longer side. If you shoot landscapes and sunsets, you'll need something around 10mm.
• Typically when do you shoot? If you shoot mostly in broad daylight, there's no point buying a f/1.4 lens. But if you shoot in low lights, fast lens might be on top of your priority list.
• Do you shoot moving subjects at low-light? Get something with stabilizer, otherwise its a waste.
• Do you use CPL filters? If yes, get a lens which doesn't have a rotating front element.
• Do you plan to upgrade your kit lens in a sense that you want to replace its focal length with something having better image quality or you need a lens which complements your kit lens's focal length reach? I always believed, if you need a better performer in the same range of your kit lens, get 17-55mm f/2.8. If you need something to complement your kit lens, decide on the focal length you need, telephoto or wide angle.
• If you shoot macro, get a 1:1X macro lens. If you're a extreme macro user, get MP-E 65mm 1:5X macro lens.

Hope this helps.