Something a little different: Trim carpentry instructional video.

[LuckyCharms]

8. When I trimmed my first door of many in the whole house trim job...

I solved the problem by using tapered wood shims behind the casing. This was how the original carpenters did it when my house was built in the 50's, because I found shims behind the old casing that I tore off.

This will tighten up the joint, but if you look closely, you can see the shims from a side view. That doesn't look very good.

Another way is to take a file, or plane, or a flap disk on an angle grinder, and hog out some material at the appropriate spot on the back of the casing, but this takes forever.

What is shown in the video is the best solution I think.