At each trial you are testing out your wedding-day hairstyle and makeup colors and application. It’s important to bring photos of what you like, and you can bring photos of what you don’t, to show the hairstylist and makeup artist what you want, or what you don’t.
For your hairstyle trial it’s important to bring any hair accessories you are planning on wearing -- tiara, headband, hairpins. You can also bring your veil to see how that will look. But make sure to have it pressed after the trial so it’s not wrinkled for the wedding. You may also want to wear a top that has a similar neckline to your wedding dress’ so you can see what the style will look like in a bodice of the same cut.
Once the style is finished and you like it -- you should feel comfortable to make comments/adjustments that are both positive and negative -- and make sure to take several photos. For your hair, you need pictures from multiple angles -- front, both sides and above. For makeup you only need a front shot but you want to write down every color used and where -- foundation, powder, blush, eyeshadow, liners, lipstick, mascara -- so on your wedding day you and the makeup artist remember what you liked.