Quit being the DM. Be the monster.
From what you've said so far, you think like most DMs. This isn't surprising - it's how the modules and books train you to DM. You design encounters based on the party strength, balancing monster ratings with party level, and then come up with excuses for the party to enter that encounter. You can keep that going for a long time but eventually it becomes tedious for the DM, who is more a calculator than a creative writer, and boring for the players, who know that every encounter is calibrated to their characters.
What's the alternative? It's simple. Start with the monster . . . something well beyond the capabilities of the party. Give it the resources befitting such a creature - not just a lair but territory, not just treasure but an arsenal, not just minions but an organization. Give it an appropriate set of goals (and here's an important point, give key minions their own goals, not all of which are necessarily parallel with the boss's). And give it a few enemies, with their own organizations and goals. Once you have these basic concepts fleshed out in your mind and on a few spreadsheets, stop being the DM and start playing the monster.
As the boss, deploy your resources through your territory, keeping your goals in mind. Create strike forces and put them to work - perhaps the players end up involved in their attacks, perhaps they just hear about them as news and rumors. Design defenses for the territory and man them appropriately, using only the resources that you already decided as DM were available. (In other words. quit making up the numbers according to what balances the encounter.) Come up with a general plan for how the boss is going to accomplish his goals.
When game time starts, your players should quickly figure out that the encounters are no longer calibrated. Some encounters they will just blow through, while others they will have to run away from. (This can be hard for players that are used to every encounter being winnable.) It might take a few sessions before they realize that they have to figure out ways to win "unwinnable" encounters. The thief will have to use his sneaking skills to do more than just get in a surprise round - he'll have to gather intel to bring back to the party for strategizing. The players may have to find new allies (which means more role-playing opportunities and larger stakes in those encounters). They may learn to set traps and ambushes that get more than just a round of surprise. (Be prepared to expand the price list beyond caltrops, holy water and hunting traps and the few poisons listed in the DMG).
One lovely side effect of DMing with this mentality is that even the easy encounters will make your party nervous, simply because they can no longer assume that every encounter is winnable. I've seen my players (@3rd level) run in terror from two goblins, just because the goblins weren't supposed to be there and they thought that their entire plan was already compromised.
When you first start, you might have some trouble spreading the boss' resources realistically. There is a tendency to put everything in one central lair. But that's not a great way for a boss to work - there are good reasons why armies have multiple bases, kings have multiple castles, corporations have multiple factories, etc.. Think of all the different activities the organization has to accomplish - not just plundering and evil, but the procurement of supplies, feeding of minions, meeting their spiritual needs, engaging in trade and diplomacy, even recreation. A lot depends on your boss' goals, but he'll certainly have a lot more going on than will fit in one dungeon. And each of these activities can become a target for your players - or the means of infiltration.
One final warning: this method isn't for every DM. It requires lots of work up-front, which is balanced by the fact that it takes very little work from game-to-game. It is also not for every set of players. Not all player groups really want to make plans or deal with the inevitable failures that occur. You will have some games in which the players simply roll through everything, and some games where they just butt their heads against the wall. But when they finally take down your boss, the satisfaction that they'll feel will be better than any you've seen in a RPG.