Hair and makeup is crucial on your big day in the sense that you obviously want it to look perfect for pictures. Keep in mind that you don't want your hair and makeup too drastic from your everyday look. You don't want to look back at pictures and regret how you did your hair and makeup because it was too "extreme" or "outlandish." It is highly recommended that you have trial runs and test out a number of different looks so you know what is best for you. You should plan to have trial runs 1-2 months before your big day, depending if and when you are getting bridals done. If you want bridals done with the same look you're going to have on your wedding day then you may need to have these appointments earlier. You can even plan to go from your trial run to your bridals to take advantage of your beautiful hair and makeup.
You should also start collecting pictures (I recommend Pinterest for easy organization of these ideas) now and save them in your wedding book to show whomever is doing your hair and makeup what you like and the look you're going for. When you make your appointments make sure that the stylist or makeup artist is available on your wedding day. If your ceremony is at 10am make sure they are open to do your hair and makeup early. It is very important you go into the trial run appointment on time so that you can see how long it will actually take to do your hair and makeup.
Some brides forget that when you get your makeup done to still look natural and yourself. You wouldn't want to do a dramatic smokey eye when your everyday look is usually mascara and a little bit of eye shadow. Your husband won't even recognize you! Makeup artists will tell you that especially on your wedding day you should look naturally beautiful and not have a lot of eye makeup. Yes, go get some fake eyelashes or go get extensions if you want, you should go all out for your day but veer away from being overly makeup dramatic. You wouldn't want to take away the attention from your beautiful dress now would you?
Depending on where you get your hair and makeup done keep in mind that it can get pricey. Most places will charge your for the trial run. Some hair stylists will even charge per "look" as you change styles from up-do to down, or curly to straight. If you have a friend that is good with hair and makeup that is definitely the best route so you don't have to break the bank. It even gives your friend a great opportunity to be apart of your wedding day, and I'm sure they'd be honored to help. Some brides even do their own hair and makeup. If that's you, more power to you. Whatever you decide remember to look natural and know that you will look gorgeous no matter what you do.