Cory, do a mixture of both. What I usually always do is modify any pre-made adventure into my own setting. A lot of times I will just use the maps from a pre-made adventure and fill it in with the content I want to use. Or just change a few things around so if a player has already adventured in that module they may be in for a nasty surprise. I do a lot of adventures on the fly; making it up as I go along. With my players one never knows what they may desire to do next or where they may go. So I grabbed all of the random dungeon making charts out of the DMG (3.5) as well as the ones from the 1st edition DMG (more detail in my opinion) and threw them into Excel. It is a bit of typing but once you have it all finished it tends to speed up the creation time of dungeons and other adventures. So you are you stuck just staring at a blank screen and wondering what to do; turn to those pages and roll some dice for some ideas. Sometimes the combinations that are randomly rolled can lead to some pretty interesting adventures. But if you "own" the world and either craft everything in it or modify pre-made modules you have control over what is in your game. Create a sheet in your campaign and when you come across interesting tokens during your searches, or graphics on the web, copy and paste them into that sheet. I have several sheets labelled names like, "Token - Fantasy" and so on. Best of luck!
