I use the "butt soldering" method almost exclusively when I have to solder a connection... usually called a "Western Union Joint", named after the splice method used on the old telegraph wires. Even without solder, if properly twisted, it's a very hard to pull apart joint... it almost tightens onto its self if done properly... however, we always solder it, right? Of course. After soldering, follow with shrink tubing (you did remember to put the shrink tubing over the wires BEFORE twisting and soldering, right? Of course)

Side note, if you ever have to use electrical tape, use 3M Super 33 electrical tape. There really is no substitute. As an example of how good this stuff is, I had a misfire on my Accord that I traced down to the plastic boot going into the top of the valve cover down to the spark plug (dude, it's a Hemi)... the spark was arcing out of a tiny hole in the plastic where it was cast... I wrapped it a few times with Super33 and the car hasn't misfired in 2 years. I did buy a new set of wires, but I wanted to see how long the tape would last.

If you have to use butt connectors, ONLY use the heat shrink sealing kind... and a GOOD pair of crimpers, NOT the $5.99 AutoZone turds... get yourself a nice pair of Kleins or a ratchet crimper like the Ideal CrimpMaster. If installed properly, they do not come apart, corrode, or loosen.

Posi-Lock connectors are very good too... they hold very tightly and are reusable. I use them for temporary connection or connections that I may need to remove later. There is a sealed kind available, but they are bulky... just use some bulb/connector grease on the inside of the Posi-Lock before assembling and all will be fine... heck, you can shrink wrap it too if it makes you happy.

Disclaimer: You may not agree with me, but I have been doing wiring on cars, bikes, trains (the small kind), computer networks, phone systems, alarm systems, etc. for many many years. I do claim to know what I am talking about.