an increase in atmospheric pressure would help... not sure how much of an increase you could go for before the large wings become more of a hindrance than a help. probably a higher oxygen content as well. but even gliding tho with large wings the larger you go you're going to need strong shoulder joints and muscles (or possibly some sort of joint-lock) to deal with the pressures that are going to be exerted.
a couple things you may want to look up on are wing suits and gliders.... interestingly we can get to really lightweight gliders that can carry a person;
Ruppert Archaeopteryx - Wikipedia
another thing you may want to look into is wingsuits- with wingsuits, the glide area that's connected to the arms has a limit, if it gets too large or long the pressure becomes too much on the human and the arms would be pressed back behind the back... which can give a rough idea of what kind of forces you are dealing with.
for what birds that have to jump off a high place i don't think there are any flying birds that aren' capable of doing self propelled power takeoffs.... however there are some that do come close, such as the condor which can't do powered flight for very long and mostly rely on thermals to stay in the air.