Feel stuck in a stylistic rut? Even in standard tuning, three-note major chords are available all up and down the fingerboard on the D-G-B strings (giving you, in the key of G, the I of the scale played open or at the 12th fret, the IV at the fifth fret, and the V at the seventh fret). If you want to throw in the minor VI, G-B-E open (or at the 12th fret) gives you an Em, and with these in your pocket you can shift the intervals up and down the neck to give yourself a little something in any key.
Open tunings can be fun to use, though, and often give you a new perspective on the fingerboard, one that is certainly suited to a lot of slide styles. Open G (D-G-D-G-B-D), open D (D-A-D-F#-A-D), and open E (E-B-E-G#-B-E) are among the more popular, and all give you a full major-chord I-IV-V progression in the open, fifth fret, and seventh fret positions respectively, as well as some useful positions for solos and fills. To make the most of these, though, you will also usually need to learn your way around some alternative fretted chord positions, too, as combining slide and standard playing is often a part of the more effective slide players’ arsenals.
The final question most novice slidesters have to wrestle with is “which finger?” There’s no conclusive answer to this one, and great players have used any of the four. Placing the slide on your pinky perhaps gives you easiest access to standard, fretted chords when you’re not playing slide, and also allows you to fret strings behind the slide (while hitting slide chords) to create minor chords and other interesting effects. Plenty of outstanding slide artists also use the ring finger, and achieve similar results. As with your choice of material, slide placement is really a matter of experimentation for each individual player. Slip one on your digit of choice, slide it up the neck, and see what you come up with.