Right hand muting (assuming you’re right-handed) is a major part of playing slide. First, put the pick away and use your fingerstyle techniques when playing slide. This way your right-hand muting is as easy as putting your finger back on the string that you don’t want to ring out anymore. When playing a riff that uses two strings, I will use my thumb and my first finger to play, then put my finger/thumb back on the the string to mute those strings, as needed. You can add your second and third fingers later, if you need them.
As for the left hand… Because the slide is a straight line, it’s functioning as a “moveable fret,” so when you want to play any more than one note at a time, they have to occur on the same fret. Looking at your guitar in standard tuning, you see that you have a G chord in your open 2nd, 3rd and 4th strings (B-G-D). This is now a moveable major chord you can slide around the fingerboard and you can play any two or three notes within it. The 1st, 2nd and 3rd strings give you an E minor when played open. With these basic major and minor chord triads, combined with your knowledge of scales, you have a really good starting point.
This brings me to tunings. Slide guitar and open tunings go hand in hand. Open tunings give you entire chords on the same fret. That’s the great advantage of open tunings for slide guitar. The disadvantage is… if you aren’t familiar with that tuning, you have to relearn your fretboard. But this always gives you a chance to look at chord chemistry in a new way and you’ll stumble on new riffs as you experiment.
The two main bottleneck guitar tunings (shown here as strings 6-5-4-3-2-1) are:
Vestapol (open D): D-A-D-F#-A-D
Spanish (open G): D-G-D-G-B-D
Spanish tuning is the open tuning that Keith Richards uses for many of the best Rolling Stones songs (except he uses 5 strings, leaving the 6th string off completely). Leave a guitar tuned to G and play around on it a bit each day. There’s a whole new world in there to explore. If you have a guitar with high action, here’s the perfect use… then again, if you need a reason to buy another guitar, old plywood acoustics are cheap and plentiful!
I’ve been thinking about slide guitar in a new way lately because I bought a lap-style dobro and have been learning how to play it. Learning lap-style has me working on a new tuning (G-B-D-G-B-D), completely different slide techniques (with a bar instead of a bottleneck) and a different angle to fingerpick. It’s coming slowly, but everyday I work at hammer-ons, pull-offs, angle bar techniques and trying to re-learn Amazing Grace and make it sound, well, graceful (or at least in tune).