Hi M29,
Believe it or not the intro to Freebird can be done pretty easily in standard tuning. Although this song has been over-played to death, you can use that intro to get some of the basic slide moves down until it sounds right to your ear. It also helps to demonstrate some of the areas on the neck where you can grab two-notes under your slide in standard tuning.
You just have to be careful when you get to the Eminor chord not to play the major 3rd (its' too easy to make that mistake when placing your slide over the 9th fret, if you hit the B string unintentionally.
The hardest thing about playing slide is playing it in tune. This takes practice. The slide is very sensitive and you just have to listen and play with it until you can hear it. It helps the intonation to use some vibrato by moving the slide back and fourth over the fret that you're trying to nail, it also makes your notes more vocal-like, which is the best part about slide playing. I also find that the slide sounds better when I use my neck pickup for some reason.
Any basic blues tune works pretty well for trying out your ideas too. Here are some more sweet spots with standard tuning for blues tunes:
Let's say we play a blues tune in A:
For the 1 chord (A) you can grab the B an E on the 5th fret and make licks from that spot.
You can also play the A and D strings on the 7th fret. This is the root and 5th of that A chord.
When you get to the IV chord (D) then you can plat the D, G, and B strings on the 7th fret. This is the D major triad. It sounds very good over this chord, naturally. If you want to get a D9 sound as an alternative, just grab the G, B, and E strings on the 5th fret
When yo get to the V chord (E) then you can play the D, G, and B strings on teh 9th fret to get an E major triad. Or, you can also use the G, B, and E strings on the 7th fret to get an E9 sound.
The best thing to do is to listen to some Duane Allman and just try to grab a lick, or two. Statesboro Blues is a fantastic tune to listen to for the text-book Duane Allman style of slide playing. Just remember that he uses an open E tuning so it's hard to copy everything he does with standard tuning.
I need to start getting into open E tuning and see what I can learn.
Good luck with the slide playing, -- Jim