Buying an instrument is an investment, and not one to be done in a hurry. Before you start looking, you need to do some thinking on why you want to replace your current instrument and what your plans are for the new one. Is it because your instrument needs some major repairs and you'd rather put that money towards a better quality instrument? Is it because you're serious about studying music through your chosen instrument? Is it because of a remark that a classmate or teacher made about your current instrument? What do you want out of your new instrument? Do you want to just play through high school and then occasionally for fun afterward? Do you want to make music a consistent part of your life while working in another field? Do you want to study in college and become a professional? I know these are a lot of questions, and not all of them need a concrete answer, but these are the questions you need to think about before deciding when and what to buy.
Note: It is important for you to know that a new instrument will not necessarily fix your playing problems. Equipment can help, can enhance, but it is frustrating and costly to constantly be on the search for the instrument, the mouthpiece, the barrel, etc. that will suddenly make you sound great. And never feel like you have to hurry - there will always be a sale, and there will always be great instruments available. Take the time to find what fits your needs and your budget.
Once you have thought about what you want out of a new instrument, you can start looking for it. But before you run to the nearest retail store to buy a brand new, never-been-played instrument, consider buying used.
I know, you're thinking, "What?! If I'm spending money on a new instrument, I want a NEW instrument!" That's fine, but there are lots of reasons to think about going with a used instrument, even for experienced players. For example, one of the most highly sought-after saxophones was made by Selmer in the 1960's. Seriously. Look around and you'll see these instruments easily get over $15,000 for an alto. (By the way, if you ever happen to find one marked "Selmer Mark VI" at a yard sale, snatch it up!) As for clarinets, some musicians argue that the process they use to treat wood nowadays makes the wood less stable and the tone less rich. Opinions of flutes vary widely and mostly depend on the maker. A master craftsman turning out beautifully made and highly regarded flutes may no longer be living, which limits your option to buying used.
I'm a big advocate of buying used, but I understand if you want to try a brand new instrument, too. Have at it. There are lots of very excellent instruments being made today. However, be prepared to spend quite a bit more money going this route. It's rather like buying a brand new car - it's generally much more expensive, but will likely not hold its value over time. The market is just so saturated with quality instruments.
Either way you decide to go, it's always best if you can try out the instrument first. Pack up your instrument, mouthpiece, ligature, reeds, and a couple books or pieces of music you've been working on and head down to the store. (Plan to be there for over an hour.) Look around at all the shiny options (very fun!) and ask the salesperson where you can go try them all out. If you're ordering online, check into the store's return policy and "re-stocking fees" so you can adequately try out the instrument before buying. In the practice room, assemble all the instruments (including yours) and decide what you are going to play. Start with your own instrument: play a scale in long tones and in pattern, then pick another and do the same; play the chromatic scale so you can feel the entire instrument; play a slow, lyrical piece, then a quick, technical one; play high notes, play low notes. Play with the intention of testing the instrument, and take note of how it sounds and how it feels. Then, move on to the next instrument and do the exact same thing. The only variable should be the instruments - otherwise, comparing them becomes difficult. Go through all the instruments in the same way. What did you think? Compared to your own instrument, what did you think? Which instrument did you like the best? What in particular did you like about it? And then comes the big question: did you like that one instrument enough to warrant spending the money on it?
Congratulations - you've found an instrument you really like. This next part may or may not be very popular, but here it goes: Now that you know which instrument you'd like, start saving up for it. As with many large purchases, when you buy using an installment plan, you will end up spending more than you would have by buying it outright. Also as with many large purchases, you've got more negotiating weight if you can write a check for the whole amount right then and there. Even better, have cash in hand. If sellers get a glimpse of a stack of cash in your hand, they're much more likely to come down on the asking price of the instrument so they can make a complete, immediate sale. Put off the purchase for a little while and save up for your new instrument.
Parents - Your child may be very talented, may be very dedicated, but take time to think about which instrument you choose. Your 7th grader still has five years to go before they decide their college major, and as avid about music as they seem now, a lot can change while they're going through high school. The point is this: a high-end professional model instrument may not be the best choice for your middle-schooler. This isn't at all to diminish the child's potential, and if you have a private teacher recommending instruments to you, please take their advice. But the salesperson in the store is there to make a sale, no matter how great the company is; they don't know your child and his/her needs as well as you do. So it's up to you to set your budget and to purchase an instrument that suits your child's stage of learning.