Birthdays are special, obviously, so you want the flowers that you send to also be as special. Sending flowers for someone's birthday should show not only the person to whom you are sending, but also the relationship that you have with that person.
Before deciding what to send, think about the other person and what he/she means to you. If you are sending flowers, just as a matter of job security to your boss, then you can allow the florist to make a selection for you, and forget about it.
If you are sending birthday flowers to a loved one, someone who is special to you, then you have to take the extra time and thought and send, not only flowers to celebrate a birthday, but flowers that are a statement about your friendship with the birthday person.
What do you want to say? For someone who is very close to you on a romantic level, pink or red roses represent love. Many people have come to look at friendship combined with romantic love as something intensely personal; some will send a bouquet of red roses with three yellow roses for past, present, and the future of shared friendship.
For sending flowers to someone with whom you have a lesser, but still important relationship, such as your grandmother or a sibling, talk to your florist and ask for suggestions. Many florists have books you can look through, so you can choose one without have to think about it; most of these "template" arrangements are just fine, and you can (usually for a small added fee) add something in, to make it more personal.
If you are having the flowers delivered (if locally), you can even bring a small gift or trinket that symbolizes something significant to the recipient. The dollar tree is chock full with tiny trinkets which may mean nothing to someone buying them from that dollar store, but added to a birthday flower arrangement will let the birthday celebrant know that you really are thinking about him/her.
Think about what makes birthday flowers perfect. You want the flowers to be delivered nicely (fresh, visually appealing, etc.) and on time. Be as specific as you can when ordering. If local, make sure the florist will deliver them on time, to the correct address, etc. Confirm that the person to whom you are sending, will be where he/she is supposed to be at the time of delivery. If you are sending them to the person's workplace, confirm the address, etc.
If you are sending birthday flowers to someone who is out-of-town, the florist you speak to will have a florist who shares the zip code with the recipient, send the flowers. Because you are out of sight, you really have no way of knowing that the flowers you paid for, are the flowers that were sent. Check out the reviews. See if the florist who is not in your town has a website, call them yourself and verbally impress upon them how important it is for those flowers to be perfect.
If you are planning on sending the same birthday flowers year after year, impress that upon the sending florist who may be out-of-town. This may (of course not guaranteed) make them offer better service. Call the recipient and (as casually as possible) ask him/her to take a snap shot of them with the camera phone; you can, although from a distance, inspect the flowers that you paid for.
It is very hard to select the perfect birthday flowers. Most important is that they were sent, on time, with a personal touch.