What advice would you give players, new and old, when it came to combat tactics?
Movement is primary. Getting to or away from your opponent can decide victory and survival.
Prepare. Do your homework and plan accordingly. Divine if possible, spy if necessary. Be ready for the unexpected.
Know your strengths, exploit their weaknesses. You should have a good idea what your character can do, but knowing what your allies can do and how you can assist them (or they can assist you) can multiply your effectiveness. Spotting your opponent's weak spots can mean the difference of a 2 round victory or a 10 round beat down and hasty retreat.
What general principles inform your decision-making?
In our campaign:
If it's a construct, throw the Adamantine wielding fighter at it
If its incorporeal, use Force effects
If it casts spells and at a distance, use readied range attacks or readied dispels
If it casts spells and is within melee, grapple
When in doubt, use massive amounts of energy damage
What would you count as basic, intermediate, and advanced tactics?
Basic: Meat shields in the front, Casters and Ranged in the back
Intermediate: Flank, Avoid being clustered/cluster your opponents, Set up AOOs
Advanced: Use complimenting spells and effects, match weapons to damage reduction, buff specifically for your opponent, Set up ambushes
How would your advice change by general party level: low, medium, high levels?
There's only so much you can do at lower levels. As you gain levels, your options increase exponentially. More advanced tactics are necessary at high levels, especially when facing a superior foe. At low levels, just not doing anything stupid will usually get you through the day.