The x264 encoding guide gives a nice explanation of the different options for H.264 encoding. Normally:

ffmpeg -i in.avi -strict experimental -crf 23 -preset medium out.m4v

Where you can deviate from the above defaults:

-crf [18-28]

perceived output quality (lower crf is better quality)

-preset [ultrafast,superfast,veryfast,faster,fast,medium,slow,slower,veryslow]

compression effort (slower preset is better compression)