I can only speak from my gas welding experience as i never got on with mig/tig/stick
I found that a gap of the metal thickness allows the weld pool flow as the gap causes a wicking action and almost draws the weld along. It also allows the weld to be a lot more flush. I cant remember which is called which (right or left handed welding) but i always used the method where the flame is angled to preheat the metal yet to be welded. Always take care when going over tack welds as you can tend to slow down which may cause a blow through - also dont stop if you get a blob - you will blow through. If you feel you are having to rush to stop blowthrough you are using too big a nozzle, always dip dip dip the filler rod into the weld pool never into the tip of the flame. If you have some scrap sheet practice - its amazing how relaxing gas welding can be if its going right!
If stick welding especially with a "home" welding system try to ensure your rods are dry - i mean really dry, if possible put them in a low electric oven for a few hours prior to using and if possible only remove one at a time as needed. (I used to be qualified in weld inspection many years ago). Welding rod flux coating is hygroscopic and can absorb moisture from the air. This doesnt cause any real problems when welding but will cause Hydrogen embrittlement of the transition area between parent metal and weld which can cause weakness. Luckily these problems dont occur with Mig & Tig - but watch out for cowboys, I have seen people using CO2 as a shield which can result in extra carbon forming in the weld which makes it hard but more brittle.
The Dog is on its way....