Buying a nice dress for your girlfriend is tricky.. but I've done it twice and thought I'd share.
I would not buy cloths often. However, if done right, it can be a wonderful thoughtfull surprise.
Surprise is the key. It will put your girl on cloud nine. She'll reward you!
Here's the method I've used with both my ex and my now fiance. Both of course loved the gesture and the dress.
Early summer is probabbly the right season. The warm weather makes everyone receptive to fun and you wear more dresses in the summer.
Look in her closet at her dresses now. Does she have a favorite? Who's the designer? What size are they? Mesure a few, shoulder to hemline. Mesaure the waist. Arm yourself with a pocket tape measure. Remember what colors she looks good in. Do it while she's in the shower. Keep this all a secret. Get something similar to her dresses now, but different.
Go online and start with the same designers. Don't pick the shortest dress on the sexiest model! (unless she dresses that way) do however, go for fun, not business. Bright colors are fun. Pick a color she looks good in. Hopefully, you'll find mesaurements. Read the reviews of girls the same weight and height if possible to check for fit. If you see something that catches your eye, purchase it and have it shipped to you so you can open it and inspect it. You never know, a tear or imperfection could ruin the whole thing. See if it's the approximate size you want. Most stores have liberal return policies. Find a nice gift box, put a card in there, pack it nicely and mail it to your girl.
If you can't find the dress.. print some pictures for refrence anyway.
If you did not get something online, go out shopping armed with your refrence pictures, sizes and measurements. Go somewhere decent.. not walmart. This will undoubtebly be more expensive than online.. and if you are shy it can be hard.. but trust me, the ladies in the shop will LOVE to help you! If you have a female friend with good taste, take her.
This is a lot of effort but if you pick something she likes, and have it mailed to her in a nice gift box, with a card maybe, you'll get lucky and she won't forget. Plus, she'll totally want to wear the dress out and flirt with you. It's great.
Most guys don't do this because they are scared. It's a great display of confidence and attraction towards your girl.
The few times I did it it worked great. But if it does not work out, you can always return the dress and promise to go shopping latter. No worries, it's a fun thing to do.