You don't have to worry about your penile tissue not getting enough blood (and the oxygen the blood carries, more importantly). you get erections at night, even without dreams of any sort, they are appearing and subsiding off and on every night, and this blood flow helps your little guy to get all he needs. If you are looking to improve blood flow all the time, it's not impossible, but like most general health concerns, does require work. firstly, any potential problems like heart disease, diabetes, etc should be checked by a doctor. if you are on any medication, you can ask your doctor about their sexual side effects. assuming that none of these are coming into play (and heck, even if they are!) your best solution for 24/7 blood supply is simply cardiovascular exercise. aerobics and endurance. Any exercise which gets your heart pumping strongly and quickly (and ESPECIALLY if done regularly and maintained over long periods) will cause growth of your vascular system, meaning that new blood vessels will be forming in your tissues so that more blood can reach them. Now, your aorta (the biggest artery that comes out of your heart) goes up, branches off, and comes down again. The branches at the top go to your head and arms, and the lower portion covers the rest. So, if you want to increase the blood flow to your privates, there's really no such thing as aerobics for your penis, so your pest bet would be exercise that focuses on the muscles nearby so that the arteries that lead to those areas are strengthened and branch more.
Having a larger penis does mean having firmer erections will be more difficult, because anyone can do cardio exercise, but building blood volume is a whole other issue. Don't worry too much about this, because there's so little you can do that's useful. Just be sure to stay well hydrated (drinking at least 20fl oz -1 water bottle full of water in ADDITION to the liquids you already consume per day) and maintaining adequate nutrition. Also, get sleep!
P.S. Biology/physiology/kinesiology is my area of study