There are many things to consider when shopping for a ring bearer outfit, especially if you want to reuse it. Things to consider are: child's age, personality of the child, formality of the wedding and colors of the wedding, just to name a few.
The child's age has quite a bit to do with choosing a ring bearer's outfit. If the child is young, you need to take the fabric into consideration. For instance, you do not want a stiff, scratchy or starchy fabric. The child will be uncomfortable, and will not want to wear the outfit again. Also, the ease of movement should factor in, too. You want the child to be able to move around in the outfit. When purchasing an outfit in advance, make sure you buy it large enough for growth spurts.
The ring bearer's outfit should reflect the child's personality. If the child is vibrant and energetic, the outfit should have punch of color. Also, it should have a bit of fun to it. Try adding a printed cummerbund and bowtie, for instance. After all, it's a child that's wearing it.
If a wedding is formal, then the ring bearer's outfit should be more along the lines of a tuxedo or suit. If it's less formal, then a nice pair of pants or even dress shorts would be appropriate. Also, keep in mind, what season the wedding takes place. Maybe a sweater would suffice or a polo shirt.
You should always keep the colors of the wedding tied into the ring bearer's outfit. If it's a formal wedding, then the tie or cummerbund should pick up on the colors of the wedding. For less formal, a button up shirt in the colors of the wedding would do just fine. A boutonnière can be used to help with tying in the colors from the wedding.
Now that you know how to pick an outfit to match the child and the wedding, the last thing would be how to make sure you can use it again. Make sure you buy the outfit with a little bit of room in it, so the child can grow a little without outgrowing it right away. By interchanging shirts and pants, you can use pieces of the outfit more often, too. If you purchased a cummerbund, bowtie or vest for the event, this makes a great keepsake for years to come.