How to trim a cake so it's level?
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.