it seems in all my games i tend to be 2nd or last in power rating and i tend to get attack a lot but i always tend to push them back. My question is how can i maintain my power at a good rating even though I've dedicated one of my cities to military production??

I think that power ranking considers military buildings and wonders, as well as number of units and whether they are state-of the- art or obsolete.

I guess if you're a protective that's not a big problem. When I fall behind the answer is usually to build units faster than I am losing them, or when the eventual war comes cause the enemy to lose units and cities faster than he can replace them.Perhaps you need a second military city, or an acadamy.

Keep your military up to date and units updated. Also build privateers and sink everything you can find - this counts towards Great Generals and helps with your Power rating too. Makes the AI spam caravels as well, which can shift the balance of power on AI landmasses, as they're not building land units in their coastal cities.

I found upgrading a big Frigate/Ironclad fleet to destroyers, while being expensive, put my power rating way up. Helps sink invasion fleets too......

have 1, 2 or even 3 cities producing nothing but units, all game long.

http://forums.civfanatics.com/showthread.php?t=249653

I like to have at least 1 city just building military.

The linked thread pretty much covers it. Probably the easiest way to boost power is to build walls and barracks in your cities, especially if you play as an Aggressive or Protective leader, or have stone.

Slavery of course allows units to be built much faster in high-pop cities. If you're in trouble, it can be the difference

I agree with the privateer option. You can also try to diplo a few AI wars, as this can increase your own by removing theirs.