First, I hate sheet cakes. OK, so now that I have that out here's my problem. I can NEVER get them level and I can never get the edges straight (vertically). I always do 2 layers and my cakes turn out lumpy-looking. I've tried leveling with a knife but I don't have a good eye for that sort of thing. I have thought about getting a big leveler but so many people say they're a waste of money so I haven't. Any advice? The tops and corners of my cakes need help!
I too have been frustrated with lopsides sheetcakes, it makes them look very "homemade" I saw a hint in the cake mix doctor book about using homemade baking strips. I cut 1inch strips from an old towel and wet them thoroughly then wrapped them around the pan and pinned with straight pins, my cakes came out really level. (11x15 pan 1.5 mixes in each pan). I had to do a little leveling and used the Wilton small level and my cakes are really level now. I was happy with the result.
I slightly overfill my sheetcake pans so that they hump up. Then, I use the top of the pan as a guide and cut the tops off with one of those large levelers nobody likes. 
I used to use a big serrated knife, and that worked fine, too. Then, I cover the top of the cake with the board I'm going to place it on and flip the whole thing and then remove the pan and let the cake finish cooling right on the board. It's totally even every time. And if the cake's still a tad warm when it's leveled and flipped, it will sweat just a bit and will adhere itself to the board and won't budge at all.
The Agbay leveler would solve all your woes, but it is an investment and not right for everyone. In the meantime, try the other tips and add this one - Keep the 'hump' you carved off the top of the cake and shove pieces of it under the corners to prevent droopy corners.
I LOVE my agbay and feel even though I don't make that many cakes it is still worth every penny of it.
I had a hard time even getting my round cakes to look leveled after I tort them and ice them. With the agbay you can even slice the very top after it's been torted and filled then your top layer of icing is very stright and smooth.
Yes it does cost a bit but my advice is to save every penny you can until you can get one. I've not heard anyone say they were sorry they got one.
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