Hey there Hopeless!
I can totally feel your pain. I've been playing this game for some 5 years now, and I still have ascended only once (and that during the last six months!) so you can draw two contradictory conclusions:
A) I stink at playing this game, so my advice is going to be terrible.
B) I'm actually more used to the early game than to anything else, so my advice is not going to be terrible.
Make of that what you will!
General advice:
1) Learn to be a coward. Running away from something you don't know if it's easy means you may die at a later time... fighting it now means it WILL kill you. If it doesn't kill you, you just lucked out.
2) Don't fight Sigmund, Grinder, Menkaure, Harold, Erolcha or Gastronok. Just. Don't. Go somewhere else.
3) Don't ever stand in sight of orc priests. The orc kill priority is priest>wizard>warrior>plain. Hide around a corner until the priest comes to you or run to his face and mash it in if there are at most 3 orcs.
4) Switch to a different branch the instant the place you are in becomes even slightly less than comfortable. IMHO at the point you see the lair entrance, it's the easiest alternative, so dive right in and go somewhere else when it gets uncomfortable. Juggling branches can be really good for your survival, even if it means you spend more food. Ctrl+g and Ctrl+f are your friends.
5) Stick to 2 or 3 solid/straightforward/comfortable character combos. When you are frustrated, roll a random or weird combo knowing that it WILL kick the bucket and that you WON'T care. This can do wonders for your morale.
6) Blowguns. Can totally save your day. Poison from a distance, run away while the poison lasts, rinse and repeat. Kills goliath beetles, worms and other insect toughies with 0% risk, and is useful for lots other stuff. Curare needles are golden against uniques - and humiliating, too.
7) Triple visitation. When you find a staircase down and there are no monsters in sight, TAKE IT. If there are any hard monsters, go back to the previous level immediately and exclude that staircase (ctrl+x for map mode, hit 'e' while examining the staircase tile) If there are no monsters around, explore for a little bit, grab any loot (this gets you loot earlier!), kill one or two easy monsters present, go back to the previous level and resume normal play.
Character suggestions:
Gargoyle or Minotaur Fighter of Okawaru with evocations:
The gargoyle has low HP but absurdly high AC, the minotaur has high HP, automatic retaliation and awesome combat skill aptitudes. Train the 4 essential skills (weapon of choice, shield, armor and fighting), invocations just enough to make Oka's powers reliable, and evocations as much as you want when you start finding evocable items. An evocations fighter with a couple of good rods can dish out terrifying amounts of damage even before getting in melee, and if you don't train other magic skills at all, getting rods from acquirement scrolls is easy peasy. Disc of Storms are also specially abusable with gargoyles since they have innate rElec. People will tell you to ignore shields in favour of two handed weapons, but if you're worrying about survivability, that makes little sense. Stick to shields, they're in the game for a reason. Choose a mace if going gargoyle, an axe if going minotaur. Use heroism liberally, use finesse sparingly (but don't be stingy!).
At high skill and with a good selection of gadgets, evocations can pretty much lay waste to anything.
Deep Dwarf Necromancer of Makhleb:
A classic. Spam pain until you get to XP level 3 (skip other spells for now) and when you reach XP level 3 learn Vampiric Draining. Together with Makhleb this gives deep dwarves three sources of HP gain. Then start branching into melee or making an undead army. When vampiric draining and Makhleb can't keep you in good shape, resort to the healing wand. When the healing wand is out of charges, recharge it.
Troll Berserker:
Swim swim hungry, swim swim hungry. Only with 'clawing' instead of 'swimming'. I can't tell much about this combo, but it was the first combo that carried me to XP level 13, waaaay back. Less thinking and more ripping everything to shreds with your claws (Choose claws, wielded weapons are for civilized pussies)
Ogre Hunter:
Ogres are cool, but hard to play. The thing is, if you go hunter you can choose LARGE ROCKS as your ranged weapon, and LARGE ROCKS are a wonderful panic button. Orc priest? LARGE ROCKS. Ghost, unique, or phantom? LARGE ROCKS. LARGE ROCKS can be a solution for most problems, as long as they last and are in your inventory. Put javelins to good use as soon as you find them. Choose whatever god you feel like, the hunter part is just for the guaranteed LARGE ROCKS and throwing skill. Since you've been playing ogre berserkers, you might as well worship Trog at the first altar you find, it'll be mostly the same but with the added benefit of LARGE ROCKS.
Tengu Conjurer of Vehumet or Dithmenos
Blast everything to oblivion until you die in a blaze of glory. Vehumet makes your blasty spells better and easier, and gives you MP on kills. Oh, also teaches you more spells. Dithmenos doesn't like fire, but makes you stealthier, harder to hit and at high piety may duplicate your spells when you cast them. Tengu are good with weapons too, but they are frail.
Naga of Cheibriados
Slow as fuck, and Cheibriados gets a lot of hate, but boosts your STR, INT and DEX like nobody's business, and when the Slouch ability gets online, you can pretty much nuke all the monsters in the screen (the faster they are, the more damage they take!) But you have to be very careful until your stats are all souped-up! Pick a background with ranged damage spells, then supplement them with throwing weaponry and a strong two-handed weapon, and play like a stationary fire platform. Cheibriados will enable you to dish it out with magic, ranged and melee at the same time. Just move AS LITTLE AS POSSIBLE.
Edit: Spelling hurr durr
Tuesday, 18th February 2014, 09:19 
Thanks for the concise answer. So does that mean that DCSS doesn't punish you for completely searching a level? Because that seems to be the implication of your answer and it seems to conflict directly with what Siegurt said. If it means both then I guess I am a little confused. I admit I am a bit of a completionist in that I feel the need to explore everything but if youd need to go around 2x then that isn't why I've died to an OP foe almost every other run (though I admit to plenty of YASDs.) In fact the main reason I started this thread was because I was dying to one or two mobs out of the blue who were ten times more powerful than their colleagues. This seemed consistent so Siegurt's note about stalling seemed to confirm what I was observing. 
