Good credit is calculated (by many lenders) by taking your FICO score which is calculated based upon what is in your credit report. Building credit generally means building up your FICO score.
Your FICO score is impacted by many factors, one of them is your utilization ratio of your installment loans like student loans. This is the ratio of the current balance to your original balance. To improve your score (slightly) you would want a lower ratio. I would recommend paying your student loan down to 75% ratio as fast as you can and then you can go back to $50/month.
A much better way to improve your FICO score is to have a revolving credit. Your student loans are not revolving, they are installment loans. Therefore, you should open at least one credit card (assuming you currently have none) right away. The longer you have had a credit card open, the better your FICO score gets. Your revolving credit utilization ratio is way more important than your installment loan ratio. Therefore, to maximize your FICO, try to never have more than 10% utilization on your revolving credit report to the credit bureaus each month. Only the current month's ratio affects your score at any given moment. You can ensure you don't go above 10% by paying your balance before the statement cuts each month to get it below 10% way before any payment would be due. (You should always pay your remaining credit card statement balance in full each month by the due date after the statement cuts to avoid any interest charges.) Note that there is a slight FICO advantage to having at least one major bank credit card instead of just only credit union credit cards. Also, never let all your revolving credit report a zero balance in a month, you must always have at least $1 reporting to the credit bureaus on at least one of your open credit cards or your FICO score will take a big negative hit.
If you cannot get a normal credit card, go to a credit union and find one that offers secured credit cards, or a bank that does. A secured credit card is where you place a deposit with the bank that they hold and give you a credit limit to match your security. Ideally it would be a card that graduates to unsecured after you demonstrate good history with them. For example, the Navy Federal Credit Union secured card unsecures for many people. I also believe the Wells Fargo Bank credit card (you can join if there is a family member who served or a roommate who did) also will unsecure. The reason you want it to unsecure and not be forced to open a new account to get an unsecured account is that you want your average age and oldest age of open revolving credit accounts to be as high as possible as this is another impact on your FICO score.
Credit unions that anyone can join include, Digital Federal Credit Union, the Pentagon Federal Credit Union (which offers a secured card that does not graduate), and The State Department Federal Credit Union (also offers secured card that I think does not graduate).
One other method to boost your FICO score is to get added as an authorized user on one of your parent's credit cards that has been open a long time. Not all lenders will report such an authorized user, however, ones that are known to do so are: Bank of America, Citi Bank, and Capital One. It is a good sign that it will report if they ask for the social security number of the authorized user. However, note that the Authorized User addition can have no impact if the lender is using one of the newer versions of the FICO scoring model, only the older versions reward you for the age of accounts for which you are an authorized user.
A very long term boost is to open your first American Express card underwritten directly by Amex such as their Zync card which is pretty easy to get. The advantage of American express is that they remember the date your first credit card was opened with them and if you open new accounts in the future they will back date the date of their opening to match the date your first card was opened. If you let your membership lapse, be sure to record the account number and date opened in your personal files so that you can help them locate it again if you reopen as they can have trouble if it has been on the order of ten years or more.
Finally, note that the number of accounts opened in the last twelve months is a small negative mark on your score (along with number of inquiries), so if you open a lot of accounts all at once, in addition to bringing down your average age of accounts, you will also get dinged for how many were opened in the last year.