Choose a room microphone with a large-diameter condenser if you're recording the tuba as part of a large band.
Use a cardioid microphone of the same type you'd utilize to record a bass guitar amp or the bass drum in a drum set if you are individually miking a tuba to isolate it.
Avoid the temptation to use a clip-on mike attached to the tuba's bell. Internally transmitted sounds of the tuba's heavy-duty valve mechanics invariably transfer to any microphone hard-mounted to the body of the instrument. The only connection between the tuba and the microphone should be acoustic.
Start with a microphone mounted on a boom approximately 2 feet above and slightly behind the bell of the tuba.
Point the microphone slightly off-center to the middle of the bell. Tubas are essentially omni-directional and benefit from mixing and harmonizing with other instruments. Close-miking a tuba like a more directional instrument such as a trombone deprives it of some of its warmth and resonance.
Use acoustic dampeners or makeshift replacements such as heavy moving blankets to reduce reverb if recording the tuba in an acoustically open room such as a church.
Choose tubas with upright bells when possible for recording purposes. Though front-facing tuba bells, such as commonly seen in high school bands, were once called "recording tubas," the name dates to a bygone era when it was necessary for tubas to project their sound forward to drive primitive microphones. Given today's recording technology, tubas benefit more from upward projection, which takes advantage of the acoustics of the concert hall.