Here's a technique I'm messing with right now to do some ambient loop stuff, but it could be also useful for sustaining single notes (or extending percussion tails/reverb), looping without abrupt start/end clicks. Only thing is that you need heavy compression/vinyl sim on your resample to make it work.

Also, you need 2 samplers to make it work. All the work is done on one sampler and then the signal goes to the RCA ins of the other. I'm using two 404s but you could use 303s too, and I never messed with any other SPs but prob two 555s would work too..... or any combo.

Let's say we have some trumpets and they go

DA-DA-DA-DA-DAAAA

and you want it to go

DA-DA-DA-DA-DAAAAAAAAAAAAAAAAAAAAAAAAAAA

and you try timestretching but it sounds awful.
Keep the first 4 notes on one pad (or chop them up, whatever), and the longer note on a separate pad, then sample (NOT resample!) the longer note from your OG sample source again, this time at a much lower level, and put it on the pad next to the louder one.

Now, make sure that the 2 are the same length. This is easy, just hit both pads simultaneouly and listen to see if they drift apart while adjusting your start/end points until they loop side by side.

Now, blast the shit out of the other sampler's compressor, and open the EXT source up.

Now it's time to "resample" (sample from one SP to another, in this case) your long note, and keep it going for however long you need it to.

put gate/loop on both, and hold the quieter one first. Then, try to hold down the louder one at the exact halfway point of the other, so now you have them following each other at the half (1/2 way through pad one, pad 2 starts and vice versa)

make sure that the louder one was the last pad pressed, because otherwise you can't control its velocity with ctrl knob 3 (start/end button lit)
As you listen to your performance of the two pads, try turning the velocity down to zero when it is about to loop, and turn up the velocity while the quieter one is about to loop. The compressor will do its part, ducking the sound behind each other neatly. just get the velocity change down as a rhythm and you can keep looping seamlessly.

Of course you can also make 2 copies of the same sample and fade your resamples using sub22, but i find it faster to do it this way.

PHEW!

good idea, a little cryptic for my first read,
i did this once but with way less steps, i think
i just stretched the last bit and added reverb,

I also do that for loops that have voice coming in 1 beat early, before the next bar starts. other wize i just chop that beat out and double the second last one.

sorry, probably equally cryptic.

I think I know what you mean, like right before the instrumental loop is about to end, the singer jumps in so you loop the end part twice to get it out?

yes exactly. it usually leads me to this weird fantasy, that there is a secret way to write music starting 1 beat early, my homey actually makes his music like that and it boggles the shit out of me, I start at the first beat pretty tight to the hit, i have seen people leave significant air at the begining or even start the loop on the snare, which I dont understand, i know 3/4 time and 4/4 time but I ALWAYS. start with a kick.

sorry for drifting off topic of your technique.

Personally, when I want to make a legato/long version of a staccato/short sample, I usually:

1) smash it with reverb

2) resample it, a then

3) add "glue" (e.g. vinyl sim) to all the related samples (e.g. horn hits) so they have the same timbre.

P.S. There are also a couple ways of making extreme-time stretching work:

1) if you don't mind abusing the sample that needs to be stretched (and, to match timbres, also the related samples that don't), then you can throw on the isolator (or EQ) and cut the highs until the clicks are eliminated
(it's probably wise to cut the lows too and just commit to a heavily-filtered sound, unless you are stretching a bass in which case you are golden!)

2) I forget! I'll try to remember soon! Sorry...