Seagulls will be far less likely to bother you if you can cause them to think that there is a predator on your porch; putting up something that is brightly colored with a black dot in the middle (which will look like a large predator with a large eye to the seagull) will keep them away, and usually – especially if you are in a beach town – you will have no problem finding something like this!
Rigging something that catches the light and reflects it at the seagulls is another good method; something worth considering trying is filling a large plastic sandwich bag with water, putting a crumpled up piece of tin foil in the water, and hanging this on the edge of your porch, as this will actually go a long way in fighting your seagull problem!
And making sure you are doing nothing that will overtly attract seagulls is one of the biggest things for keeping seagulls away from your beach house and your porch; do not leave food unattended on your porch, and do not throw food out in your yard for the seagulls to enjoy! You will be more likely to deal with seagulls if you give them incentive to come your way, so minimize their incentives to visit your house and you will minimize your problem!