Here's A Way To Play Blood Bros. For As Long As You Want :
1) Get to the second screen (preferably without dying on the first screen).
2) Shoot the building on the right side of the screen and start shooting the trees in the background on the right. 20,000 points fly out from each bunch of trees. Continue.
3) That is basically it. A few problems occur and here is how to counter them :
3a) 5 men run across the back from the left and throw grenades : Just shoot the grenades before they get to you. Eventually they come out less often, and will stop altogether.
3b) The men on the right come out from the edge to shoot you : Same as above. They stop eventually.
3c) A plane flies over and drops bombs : Same as above. It stops after some time.
3d) After a certain amount of time a bird comes out to kill you : Let it. Only 2 birds ever come out (sometimes only 1) and then they also stop coming. This is why it pays not to die on the first screen.
4) Hold your finger on the button to keep shooting the trees (sticky-tape works well here) and prepare to get 'really' bored. Play the neighboring game so that you can keep an eye on it.
Extra Live : On the 1-1 and 4-1 stages, the tin can be shot eight times to give you a free man. If you miss it, don't bother trying again - you only get one chance.