The last settings used are stored as the default. For Premiere 5.5 and higher, the "Match Sequence" setting is very useful. When this is selected, Premiere will look at the original source and output the file using the native format codec and size. Whenever feasible, use this option to export the highest possible quality.
In the "Format" drop-down, select H.264. Premiere will select the program setting that most closely resembles the source. Use this option if it is not possible to export a file in its native format, due to size considerations or because the end user or playback platform can't accept the native format of the footage or sequence.