Picking out what to wear on a regular ol' Tuesday can be a least-favorite-activity for some.
Picking out what to wear on a Tuesday where I'm paying someone to take photos of me? Eek.
Let's start with the why. Why are you having photos taken?
A} Because you often have your picture taken and this is the norm for you?
B} Because you're excited to look back at this special day?
C} Because you need new photos for the business you've started?
Option A: Wear what represents you best at this moment in time! If you're lucky enough to document moments of your life on a regular basis, make each time special and unique! Rock that funky Anthropologie dress you spent your paycheck on.
Option B: Engagement photos, family portraits, self portraits, etc. Go for timeless. Matching white t-shirts and jeans on a beach? No... Timeless and classic can be up for interpretation and that is a good thing. Go for your version of timeless, something you'll be proud to see in 10 years, or at least a appreciate.
Option C: Photos to use in your business endeavors. First, consider all the details of your business: logo, color scheme, first impressions. If you're building a website, you want it to look cohesive and that includes your outfit in your About Me page photo. This can include your uniform, of course!
When all other efforts fail, go for flattering. Wear what flatters you. Always!