How to trim a cake so it's level? If you cake is too domed, you can just cut the top off, right?

Thing is, I tried this, and now my cake is taller on one side than the other. So it's still not flat! ;-)

Does anybody have any good tips for keeping the cut level?

(I already know you need a knife long enough to go right through the cake. I have that. It's just difficult to cut in a straight line by hand.)
Mostly it comes down to practice. However there are some tips and tools that can make it a lot easier. If you have a cake stand (the kind that spins) then you set the cake on the stand and slowly turn it with one hand while cutting forward and back with the other (trying to keep the cutting hand as level as possible). Another option is to buy a cake leveler http://m.wilton.com/store/site/product.cfm?sku=415-815 - if you make a lot of cakes it can be very handy.