I love leather/suede straps. Nylon slips way too much! I never take my strap off my bass. Don't feel comfortable just holding it in my lap, basses can be slippery little fellas and thump onto the floor

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Definetly practice standing up if you're gonna gig standing up. Practice in the semi dark (or even in the dark). I found out the hard way that most gigs are in nice dark corners.......and when you're used to being able to see the frets and all of a sudden you can't it's trouble! I put some of those glow in the dark fret markers on my bass because of this

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Also, check the angle you hold the neck at. I realized the other night....as I'm playing....that my bass had slid a bit on me and was leaning towards the floor a little bit (instead of angled up a bit towards me). That's all good until I went to play I Will Survive (cake version bass line) and got out of hand position and started the pattern one fret down (gack).. when I glanced down to fix my position I couldn't see the top 3 strings and had to real quick lean back and reset my bass. lesson learned...I'm looking at the fretboard too much for that song and gotta practice it with eyes closed a bit.
Practice your songs under the worst possible conditions because that's what you might run into at a gig.... standing up.... squashed in a corner where you have to angle your bass up more than normal... dark corner...bright flashy lights blinding you so you can't see the fretboard (effect lights always manage to be pointed my way)...turn off your amp and play without it.... build strength in left hand so if you have to you can keep playing just the left hand and use the right to reach over and replug in whatever cable a "patron" stepped on... or so you can adjust your eq when you have to...or so you can hold singers guitar up while a slipped strap is being put back on while they're singing (happened!)... bass players motto is *be prepared*

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Plus, sitting down, bass tends to angle toward you so you can see the whole fretboard and standing up you can't-leading to variations on above scenarios of 'can't see"

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