How can I be an impressive DM to ensure enjoyment of the group?
Avoid problems with only a single solution
Do you remember those old video games where you'd point & click your way through it? Games like Myst? I believe they call it "pixel bitching" when you have to find juuuust the right pixel to click on. I remember doing it in the Baldur's Gate games, before the enhancement that highlighted what you could click -- as I moved the characters through a forest, for example, I'd randomly move around the cursor, watching to see if it changed. If it did, I'd go back and hunt down that one pixel that linked to a hidden magic item. It was fun the first time you discovered something, but after that it was just, "Really? I have to do this now? I have to find just the right pixel? Ugh."
DMs do that in games. They are so proud of their riddle, they don't have any alternative if the players can't figure it out. So the game grinds to a halt while players try to find just the right answer. Or the DM assumes that the players will fight the villain, but when the players find an entirely reasonable (and unexpected) way to negotiate peacefully, the DM forces the fight even though it plays out as contrary to rational thought.
Thus, you should read about the
three clue rule.
Favor many small skirmishes
When a DM tries to balance a fight, keep in mind some tips. First, combat is chaotic, so some tough fights are going to be easy, and some easy fights are going to be difficult. Deal with it. Don't game the system (much). If you do, the players may notice and feel that no combat is "real." At that point they'll stop being invested in the outcome. Sometimes, PCs have to die because things didn't go well. Same for villains.
Second, keep in mind that not every fight is supposed to be a fair, equal fight. In fact, the majority are supposed to be one-sided, if the DMG's advice is followed.
I played with a DM who assumed that every fight should be an even match, and if it ended with the last bad guy dying just as the PCs were at 1 hit point and on their last Cure potion, it was a great combat encounter. Epic. Turns out, those fights played out mostly just as slogs. We grew weary of having to sleep after EVERY fight, just to regain life & spells. We grew weary of being out of loot because we ran through wands & potions at an incredible rate, just to survive everything.
When it got bad enough that we started metagaming about suspiciously inflated enemies -- "wait, we've done 140 hit points of damage on a perfectly ordinary
tiger and it's still coming at us??!?!" -- we knew combat wasn't good. The slavish adherence to everything being life or death not only wasn't fun, but isn't how D&D is intended to play.
In addition, it turns out one of the big complaints about D&D is lessened if you run combat as intended (a few easy encounters followed by a tough BBEG). The super tough fights cause players to scrutinize and hyper-optimize every decision. The combat drags out, which is a typical complaint for 3.5 edition, especially at higher levels. However, if you throw at them what is considered to be a "normal" encounter by the DMG -- something that uses just a fifth of their daily resources (in other words, leaves them with hit points to spare, spells to spare, and equipment to spare), the combat will go MUCH faster.
When my old DM would run these life-or-death combat encounters, we'd typically get through two in a five hour session. Yet when I ran a game, we'd get through six or seven in the same amount of time. I was not better at the game. In fact, I had a
worse grasp of the rules and sometimes had to stop combat to refer to books. However, I was throwing four encounters at them that were typically a CR
below the average level of the party, and then one tough fight to wrap up. Contrast that with the other DM throwing EL 10 fights at us when we were 6th level. That
possible but when
every battle is that, ugh.
So for balancing fights, I find it's best to use one of the many free
encounter calculators. That calculator will show you how hard a fight is -- look at the "difficulty" field near the bottom after entering your data. 80% of fights should be "easy" or "challenging." Only 20% should be "very difficult" or "overpowering."
For a group of four sixth level adventurers, that means that
one ettin is a normal fight. Or two owlbears. Or four worgs. "But wait," you might say, "those fights are probably kinda easy!" YES. Such characters should be able to have 3 or 4 or 5 such fights in a day before everyone says, "let's rest."
Don't get overloaded by world creation
Finally, the advice I personally give over & over again is to follow just-in-time build processes, or lazy loading engineering practices. That is, just build what you need to deploy during the upcoming game. If the party ends a game standing at an intersection that will lead to one of four towns, you do not need to stat up all four towns and everyone in those towns. Instead, you need to stat up one. They will only explore one of the four towns the next game, and whichever direction they pick, they get the one you made.
Some people say that's cheating and diminishes player choice. However, a few things. First, screw player choice if it burns you out and ends the campaign. Second, screw player choice if you can pull some smoke & mirrors and have the same happy outcome as some poor schmuck DM who stayed up until 3 AM for 15 nights in a row just so he could
really build all the towns before the next game. Third, it doesn't really screw player choice to do these things. Why? Because eventually they will go to a new town and you'll stat up that new town, and it'll be different from the first town you made, thus
actually causing real differentiation in outcomes, but without you needing to frontload the whole process.
Don't burn out. It's OK to be an illusionist as a DM. It's OK to stat up a monster like this:
AC: 20
HP: 30
Attack: long sword, 1d8 + 2.
Description: green & warty bogeyman.
That's it. That's the stat block. Took 5 seconds. It's legit, it works, and if someone tells you that you must slavishly create fully equipped and totally complete stat blocks in monster forge or something, well, just think about what's important to you: getting burnt out trying to hit an unrealistic goal, or having loads of fun with your buddies.