The only way to maintain a longer lasting erection without the use of pharmaceuticals is to use a device such as a "cock ring" that secures in place at the base of the penis underneath the scrotum. As far as premature ejaculation goes, typically such things are dependent on an individual basis. But, if you are having trouble withholding ejaculation during sex, then one remedy is simply to masturbate 1-2 hours prior to intercourse. Due to the natural refractory period of your body, ejaculation will usually take longer to achieve for most individuals.