Case 1: If you have purchased an Equity future for Feb expiry and if your target has not been achieved, then you can sell it on expiry day. You can also go ahead and buy the Equity Future for March expiry but the March Future will be trading at a premium to Feb Future due to the cost of carrying a position forward.
For example, currently, Reliance Feb Fut is at 1080 while Reliance Mar Fut is at 1086.
Case 2: You can also go ahead and buy the Equity Future for March expiry today. You will have to pay the Normal margin to carry this position forward.
Both these cases need not be beneficial for you.
The trick is to make sure you are making a profit. If your studies tell you that a particular stock is going to go up in the following month, then you will have to wait for a market dip to buy the Stock Future at the lower possible price. So everytime there is a correction in the stock price, it would be a good time to pick up the Future of that stock.