Depending on the format of the flat file, I would recommend one of the following:
1) A Fast CSV Reader
] - Handles delimited file formats
2) A Portable and Efficient Generic Parser for Flat Files
] - Handles delimited and fixed width file formats
Both will allow you to get the data into memory in a quick way. Depending on what you're doing with it though, you may prefer one of the methods Nijboer has already suggested.