If you don't mind white clover, try planting a mix of clover and grass seed. Clover will provide nitrogen for grass, and will stay green and keep the soil more moist during drought. The combination will fight weeds, and as your lawn becomes more lush, the clover will start to give way to grass and become less prominent. It's still soft to walk on, and you may be able to give up fertilizer for good. It's the way all lawns were done before synthetic herbicides came about which tended to kill clover.

Without a good reliable soil test you cannot easily determine what does need to be done, other than adding organic matter. These simple soil tests might be of some use,
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains� too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
but do not replace a soil test for soil pH, P, K, Ca, Mg levels and balance.

Kimmsr - my home is 5 years old and built in a previously wooded area. All of the topsoil was scraped down to hard pan before building.

We put a fence in a few weeks ago. 30 minutes of digging with an electric 2 man auger got us 12" down. Filled the hole with water and it sat for 12 hours before I quit checking.

How do I go about ammending the soil without damaging the existing lawn.? How often can I apply organic materials and at what point is it too much?

The only way to know when you have enough, or too much, organic matter in your soil is to look. That test 1 is a good way to know, although you probably will not need to repeat it very often.
How often to apply organic soil amendments depends on what you are applying and what you need to do. Do not apply so much at any one time that you smother your existing grass, but enough so a difference is made. That may be as much as 1/4 inch of some materials. Applying as often as once a month may be okay for some materials depending on how active the Soil Food Web in your soil is.