As a DM who loves adventure modules, I look at them not as pre-prepared meals, but as recipes (sometimes including an appendix full of ingredients). I'm the chef, and as a chef with over a quarter century of experience, I'm going to inject my own cooking techniques - I can spot when the recipe's baking temperature might inadvertantly burn the bottom of the pan, or that there's only about a third as much garlic as is needed (Americans don't use nearly enough garlic when they cook), or if that extra vanilla icing in the freezer might go well as a topping.
As for my general use of modules:
1. Following Ed Greenwood's advice to keep five plots going at any given time, I keep about five modules of appropriate level ready at any given time, with possible hooks no matter which way the characters go. I might not have them all "prepped", but I know how to potentially lead into them and fill time until the right place for the players to bite on the hook.
2.Modify to suit the party. Create hooks different than the ones given in the book if necessary. Recently, I was trying to figure out how to tie a wizard PC into an ongoing plot - we decided he was trying to get into the library of the keep that the characters were raiding. Bingo. There are plenty of ways like this. (In addition, I usually have at least one ongoing-NPC villain who is hunting at least one of the PCs for one reason or another - a Jabba sending bounty hunters to keep the PCs on their toes - and on the move, if I don't want them getting too comfy anywhere. Thus circumstance can lead to adventure - when you end up wandering into a cursed town and fall victim to the curse yourself, for instance. Or wake up in the middle of the night to find that your roadside inn is run by Werewolves, and the PCs are the midnight snack!)
3. Modify to suit the setting. If the module was designed for the setting in question, disregard. But otherwise... I usually change any names that sound out of place to fit the culture, swap out monsters for more region or setting appropriate ones, and make other similar changes. I run Midgard - one of the nice things about Midgard is the majority of the cultures map pretty clearly on to one or another real-world nation, so there's plenty of sources for appropriate names. Often I might just translate the name into an appropriate language, or change the NPC names to ones of the appropriate ethnicity with a similar derivation, sound, meaning, or even just the same first letter.
4. Rebalance. Often I'm running adventures for a party a bit smaller or larger, or higher or lower level, than the module calls for. Thus I run everything they're likely to encounter in the upcoming session through Kobold Fight Club to balance, and modify from there as necessary.
5. Have maps and stats ready. I'm a laptop DM, so I run all my adventures from PDFs. I use print-to-PDF to copy all maps and statblocks from the adventure and the various monster books into individual one-page files, so I can have them all open as tabs at once. Makes things much more convenient than paging through the module or the Monster Manual (even on PDF) when you're trying to keep the game running.
6. De-vanilla the magic. I like my magic weird and wonderful, and I completely reject the belief that D&D magic items can't be. In fact, it's not hard at all. Go through, and, aside from potions and scrolls, give every magic item a few special touches. Sounds. Materials. Origins. Prophecies. Minor curses. Nightmares. Shapeshifting. Intelligence. Ties to extraplanar creatures. Who knows what? The random tables for modifying items in the DMG are a good place to start... I usually roll a few of those and daydream from there. There's no such thing as a boring +1 longsword in my campaign. Ever.
7. Get to know the key NPCs. I'll usually daydream a few dramatic hooks about how to roleplay them - often simply by copying manners of characters in movies. When I was running a very RP focused module a few months ago, I started to instantly see characters in my mind as I read their descriptions and statblock. A friendly but corrupt and conniving guard lieutenant? For some reason I imagined him as Robert de Niro's "Harry Tuttle" from the movie Brazil... only with a polearm instead of a plumber's wrench. A mysterious and powerful darakhul (true ghoul) fixer who invites the party in for tea and an info dump? Morpheus from the Matrix immediately jumped in. A panicked german venture capitalist calling my office one day inspired an egotistical Dwarf arms merchant of my creation. And so on...
Just remember... the module is a recipe, not a meal. Add your own magic, and heap it on!
