Also, don't bother going for the bigger ships unless you're just taking it easy for kills and bounties. The kill reward for an Anaconda is in the region of 80-120k, but it takes a lot longer (-time), uses more ammo (-creds) and if you use shield cells or take any damage thats also bad (-creds for repair/restock).
"Don't Kill Big, Kill Smart"
At a resource site, fly slightly out of the asteroid belt, don't hang around in the middle of it too much and always keep at a low speed (don't sit and wait for stuff to spawn) - the cobras and their ilk usually spawn more often slightly out of the 'roids belt, whereas I'#ve noticed you get much more in the way of Sideys and "Clean" ships inside the belt itself.
Next, I almost religiously look at the Contacts panel every few seconds - you will see unknown Cobras appear on there muuuuuuch sooner than you would spot them in real space.
Also, look around for the tell-tale pew-pew lines of two ships in a battle. One will always be Wanted. They will often be out of range of your scanners, (>7k away), so these won't show up on Contacts.