I'd start by finding a group that studies longsword in your area. You'll generally find two "schools" being taught: Italian, based primarily on the treatise of Fiore dei Liberi; and German, most of which is in the "tradition" of Johannes von Liechtenauer (mainly taught via manuscripts by Ringeck, Talhoffer and Meyer, as all that survives of Liechtenauer is a later copy of the merkverse he used to encode his teachings to prevent them from falling into the "wrong hands." This stuff was serious business, and the politics of the Medieval fencing guilds alone is kind of fascinating). However in truth the Italians and Germans are both teaching one very closely-related system and there's not really much difference between them. This is particularly evident when you include what little is known about the somewhat fragmentary English tradition, which shares a substantial amount with the Germans. Buying modern translations/transcriptions of the manuscripts could be an alternative, and there's some available online, but it's better to have a more experienced practitioner to study with.
As for actual tips; learn the guards, and don't take a step without shifting into a new one as this keeps your opponent guessing on where your attack is coming from. One of the biggest flaws I see is when fencers stay in one guard too long. Additionally, some techniques and strikes work better from one guard than another. Cuts from the left are a bit harder to pull off from Right Vom Tag (sword hilt at or slightly below the right shoulder, blade angled slightly behind you) than from the left.
Also, be aware of the mechanics of cutting from the shoulder, the elbow and wrist; cuts from the shoulder are slower but more powerful cleaving blows, while cutting from the wrist is quicker but more reliant on slices. This is REALLY important for the longsword, as most inexperienced fencers seem stuck on shoulder cuts. Keep in mind how the hands work together as well. Your front hand is just the fulcrum when handling the sword. Much of your power and control is actually going to come from your trailing hand. Position on the grip is a matter of personal preference, though I prefer to rest the trailing hand on the pommel (my swords have scent-stopper pommels like the one on that waster pic I posted, which GREATLY facilitates this as it gives me a nice shelf to rest on. I mainly grip the trailing hand with the space between my thumb and index finger, so I'm actually barely holding on at all). This gives me a lot more leverage, and allows me to put a good amount of force even into wrist cuts due to the push-pull action.
