Entirely copied from @rpasek, 100% of credit is his!

• Use flux. Use a lot of flux. Like too much. I can't stress it enough; it is the biggest mistake by most beginners in soldering. The flux in your solder is insufficient. Flux protects your joint from oxidizing and when heated, the acid in it cuts the oxide layer off of whatever metal you are soldering to. Without flux you will get crappy looking joints that are dry and prone to cracking. It is also hard to get a joint to heat up and wet without flux. This leads to beginners bending and damaging tips and PCBs. I recommend a gel flux as they don’t boil quickly and they stick around to protect your joint at soldering temperature. Use flux!!!!!

• Don't force soldering iron tips into the material. If the solder isn't wetting, either you are not using enough flux, you don't have a big enough tip, or your iron isn't hot enough. Forcing it will damage your tips and good tips are expensive.

• Soldering is a pain in the face! Use a fume extractor or a fan if you are soldering for more than a few seconds. Flux is an acid, and the heated fumes will burn your lips, nose and eyes if you are exposed to it for too long. Take it from me; I soldered for half a day or so without a fan and I was in pain for days. The skin on my lips and nostrils peeled off after a few days.

• Clean up your work with Acetone or Isopropyl alcohol with a tooth brush. Flux residue may have small solder balls that may currently or in the future short out your work. Use Acetone if you can. It’s more aggressive and removes flux much faster than Isopropyl alcohol. Regular PCB's can handle Acetone but soft rubber cannot. Flex PCB's can also handle Acetone but will delaminate if left in it for too long. Test your material before applying.

• When using solder wick, cut small pieces off the roll. Don’t use wick attached directly to the roll. If you drop or hit the roll you will likely tear the pads you are cleaning off the board and then you will be sad. Also, use flux with solder wick.

• Magnet wire has a non-conductive coating on it. Strip the magnet wire with a ball of solder on the tip of the iron. I selected a wire with a nice low temperature coating so it burns off easily.

• When using a hot air, protect the plastic items you don’t want to remove with Kapton tape.

• When using hot air use a tip which is substantially larger than the part that you are trying to remove. You are trying to heat the board to the point that the solder melts underneath the part, meaning the temperature of the board is what really matters. The temperature of the surface of the part is irrelevant.

• When using hot air, use around 400C with a larger tip on the lower side of airflow. Small tips with high airflow will blow your parts away.

• Don't use helping hands with plastic flexible arms. They are too wobbly to do any serious soldering on.

• Don't use flux pens. They don't dispense enough flux.

• Don't use water based flux. It boils off before you start soldering.

• Don't use rosin flux. It boils underneath parts and leaves a sticky gooey mess that is hard to get rid of.