This is usually caused by excessive sizing lubricant on your case. If you are using a lube pad this is easy to do. It doesnt take a lot of lubricant, use it sparingly and try not to get any on the neck. I gave up the the water based stuff years ago and now use Hornady One Shot dry lube, it's not as messy either and comes in a aerosol can. For lubing the inside of the neck and good dry lube such as mica and Imperial Wax does great.
Couldn't have said it better. A poor man's spray on lube can be made by squeezing a tube of Lee sizing wax($2) into a spray bottle of alcohol and shaking to mix. Everytime you use it you will need to shake it, as the wax is not alcohol soluable and it will settle. Shaking it forms a suspension, but it does work. After the alcohol evaporates, the cases are ready to size, no denting. The Hornady One Shot is real nice though, it is very wet, (flows great, covers the cases fast), and is very clean to use. Sometimes you can find it on sale at Midway, or a local gun shop may have it.