Research the band that you are auditioning for. Check out live performances if you have the opportunity. Listen to the recordings. Check out the band's website. Talk to other musicians familiar with the band. If you are auditioning for a band that is being formed, investigate the former bands that the members played in. The more you know about the band, the band's music, and the individual members, the better prepared you are for the audition.
Learn the songs ahead of time. If the band has recordings, study and analyze each song. Learn your parts inside and out. Listen, study and play along with the recordings from the "Big Four" thrash metal bands - Metallica, Megadeth, Slayer and Anthrax. This helps to get your chops up to speed and puts you in the right frame of mind to audition for a thrash metal band. It also prepares you if you are asked to make the song sound more like Metallica or more like Slayer.
Practice your instrument intensely before the audition. In addition to knowing the songs, it is important to be at the top of your game for the audition. Thrash metal requires speed and precision. Set up a rigorous practice regimen a week before the audition. Work on speed and accuracy.
Show up on time with your own gear. The band may have amplifiers and drums set up for the audition to save time, but bring your equipment just in case. You will have to use you own guitar or bass guitar. Bring your pedal board with you as well. Don't be late. Showing up late ensures that you won't pass the audition. Plug the directions into a GPS unit or do a map search. Know exactly how long it will take you to reach the audition location.
Don't be a showoff. A band is interested in finding musicians who have the chops but who are easy to get along with as well. Your personality is being evaluated as much as your musical skills. Avoid talking incessantly about how much you can offer the band. Express an interest in them and take the time to talk and listen to the individual members of the band.