Hope I can help. I am in the military and hold a NASA level certification for soldering (6 week intensive courcse and annual recert.)
1st get a good Iron, dont use a "gun" with R/C electronics especially batteries a cheapo 40 watt iron can help you ruin you components as you take longer to get the solder to melt. Yes weller, hako and ungar make good irons. By the way the best is PACE we use the PRC 2000 but it cost about 3 grand! It can do anything you want incuding surface mount and repair of the inside of PC boards down through several layers although this is over kill for us. Try to get an iron that is at least capable of 800 deg. you will need this for your batteries. You only need to be on the battery for a couple seconds and dont risk over heating those expensive SANYO HV.
2nd use 63/47 solder it has a special property (called eutectic) in that it goes straight from a liquid to a solid at a very low (361 deg.) melt temp. and has no plastic point this means that you have a lesser chance of getting a fractured solder joint that can/will break durring a bad crash etc.
3rd Flux is your friend! DO NOT USE ACID CORE it will destroy electronics. the best is RMA liquid type flux (rosin mildly activated)
4th and most important is to clean all conections thouroughly contaminates or oils etc will cause the solder not to flow. resulting in excess heating to try and make it flow and damageing you components. use an ink erasor (the pink pencil one have oils in them) then use alcohol to clean the rest but dont let it dry blott it with tissue.
If you follow these steps you should be soldering like a pro in no time.
check out http://www.crane.navy.mil/2m/2mpage.htm
feel free to email me for any more? @ email@example.com