I'll pretext this with everything I say doesn't necessarily apply to anything but C80 and Peco C55....
1) Cleanlineless of everything is critical, even if it looks clean, scrape and burnish.
2) Never use old or corroded rail joiners. Not worth the hassle. Use new material.
3) I cut rail with old Lambert Rail Nippers, out of production for years, but that's the 'flush cutter' type of diagonal cutters. still takes a little file cleanup as it tends to put a burr on the top and bottom of the rail heads.
4) Unless I'm stuck beyond belief, I always try to solder up flextrack sections and THEN curve it into place to keep curves as smooth as possible.
5) When tying out to an existing section, switch, whatever: I cut flex ties off first, rough-cut one section too long letting the rail hang out, lay the rail heads beside each other, set the alignment, and THEN lightly criimp both rails at the same time; remove flex, cut and cleanup, then reassemble tje connection.
6) Slide rail joiners all the way onto one rail, butt-align the rails, then slide the joiner across the connection. Don't try to bend or flex the connection into place, it will expand the joiner.
7) You want the hottest biggest iron that is easiest to handle. I use a simple Radio Shack 30W pencil iron, not the best. If it doesn't have enough horsepower, you'll melt ties before it gets hot enough to melt solder. If it's too big, it's physically hard to handle with any precision.
When its right, the solder will flow out, not accumulate in blobs at all. Usually the biggest reason that solder won't flow is that the rail is dirtier than you think.
9) You don't need as much solder as you think
10) Always solder to the outside, not the inside, of the rails, and you don't need both.
11) If you do get unsightly blobs, use desoldering braid, or grind off with a dremel abrasive disk on medium speed as a last resort.
12) I never continuously solder rail beyond 48 inches due to expansion/contraction. I'll put in a rail joiner and solder in more feeders, jumpers, whatever, but 4' is about the limit.
13) There's no shame in using sectional track instead of flex, particularly if you're doing C80. It holds gauge better and can be soldered up. You can adjust radius on sectional track by cutting tie sections out from underneath (rail nippers again) and bending it slightly. It works great if you have to do a flextrack joint on a curve: flex>>solder>>short section of sectional with joiner>><<<short section of sectional with joiner<<<<solder<<<flex. that's the only sure method to prevent kinks on curves where you must have a rail joint in it.
14) When putting in feeders, tin a short section of rail, tin the wire... then hit the two together.
15) When possible, put feeders in underneath the rail before you lay the track. Under is always better than beside.
16) The really difficult thing is putting in fine, single-strand jumpers on any switches that have separate points. I don't trust rivets, joiners, whatever to keep a connection to the points. I'll do all the switch jumper work before the switch is ever installed. That's borderline nuts perhaps, but I've got switches that are really inacessible so I do everything I can think of to make them bulletproof.
I'm sure there's better tools and techniques, it's not absolute, but that's some of the basics I've accumulated over the years. The only thing I'll climb up on my stump and defend it to the last man is the cleaning thing. Just because it looks clean doesn't mean it is.
I've got an awful lot of railroad that was built in '83-86 that is still running just fine; no power fails, no kinks, no retro-feeders to fix problems.