The Tavern

The early game is so random and it's easy to lose a character due to bad generation or just bad drops in general. Especially for certain mages that only have a few shots to kill an enemy with their level 1 spell before they get their face eaten off.

Anything past Lair aside from a few save or die situations I can say any death would be your own fault.

Trolls tend to have a pretty breezy early game, some people do a bunch of trolls in a row. Certain spriggans and centaur classes are very good and easy, and DDs and minotaurs are sort of easy mode for a lot of people.

I don't know how people do randomised streaks though.

lure/exclude/retreat. use ranged weapons. be conscious of where/how much noise you are making. if you find something powerful for a low investment (poison in particular), use it. take any viable god early (this includes trog for most casters).

(note I may be oversimplifying something or missing something important: I am not a streaker )

You could always just actually watch and see what they do. The ttyrecs are available.

Back a long time ago when I would spectate good players somewhat regularly (e.g. elliptic and mikee) I was struck after a while by the realisation that they just plain did not get into the same dangerous situations I sometimes found myself in. The important realisation was that this was not luck; they were actually actively preventing the bad situations from occurring.

This isn't a very exciting answer but if you just play enough and get better you'll start to streak a bit without really aiming for it. My first 2-streak wasn't at all planned, and came around my 16th win. I've now gotten up to 4, again not exactly aiming for it, it just kind of develops (though game #4 in that streak was a Tr). I expect if I keep playing crawl I'll get up to 5 or 6 sooner or later. Now the players who just streak forever on random chars, that's a different story. Also, past a certain skill level I suspect most players could streak something like DDBe indefinitely if they really wanted to and had the patience.

This is the best thing you can do to improve your play. Some people will say that playing is the best way and in this case I disagree. Being highly successful at Crawl (which I am not, for the record) requires a very specific set of habits, the contraries to many of which you probably have reinforced through your own organic play. Watch very good players and see what they do, and the kicker is, compare it to what you do and ask yourself what's different.

what i do have is a very specific set of habits
habits I have acquired over many years of playing
habits that make me a cipher for players like you

I knew that I was playing too much when I got my first ACCIDENTAL streaks with SpBes I think. Then I played some more and accidently got an opas, opwn streak...

So I cant really help you with planned streaks, but if you play enough it comes naturally!

Part of getting big streaks is being willing to play in the most tedious way possible, because that is the safest way possible. But even playing naturally, once you get some practice you'll get small streaks here and there.

Playing strong races and backgrounds certainly helps. If you get a win, play the absolute easiest combo you can think of for your next game to have a better chance to streak.

A lot of 'unpreventable deaths' actually aren't. Just because being surrounded with 1 hp and no consumables is pretty much guaranteed to end in death doesn't mean you couldn't have avoided that situation in the first place. Occasionally a monster in early D just gets lucky though, when you have no escape consumables and the adder just keeps dodging your attacks, you might just be screwed. But even then, most of the time you can avoid having to fight that adder in the first place.