My suggestion would be to first buy choice rather than select, and look for one of decent size. Secondly, don't cook too low-- too low a cooking temp usually will result in dried out meat-- maybe aim for 240-250* at the grate and aim for a final internal temperature of around 185-190* rather than trying to go by time. Third, if the cut you purchase doesn't have at least a 1/4" fat cap on it, consider laying strips of bacon on top and/or basting with a mop that contains some oil to add and preserve moisture. Be sure, finally, to let it rest 20-30 minutes before slicing.
