Before I have boards made (with new footprints), I print the layout on paper and try to put the parts on that, first. That is, when I remember to do it. I've made a lot of surface mount footprints over the years, and the datasheets aren't always enough. The most difficult footprint I've drawn so far is one for a SODIMM (For this thing:
Toradex SODIMM Computer Module )