I use a small amount acid Flux on the surfaces to be soldered and a solid joint is very easy to achieve. Also, as someone else mentioned, keep your tip tinned!!!
Gepetto I am a NASA certified soldering technician and trust me on this Dont use acid core flux in the long term it Will destroty your electronics, the flux will keep eating away at the intermolecular bonds and eventually corode the leads of you "work" use an RMA type flux (rosin mildly activated) or just plain rosin this is designed for electronics where as acid core flux is designed for plumbing work. you can see my previous posts on a topic called "soldring help"
BTW if anyone has questions on soldering to any level or needs help/advice feel free to email me. This basicly my living for the moment I use my iron for no less than 4 hours a day minimum usuallly longer depending upon the type of work going on, I solder stuff for the military that goes into fighter aircraft etc. so I can honestly say my work holds up to the most extreme use.