The appraiser looks at sq. footage, basic amenities, and then finds comparable's in your neighborhood.
Ideally they are looking for a house just like yours within a few blocks that has sold within the last month. Now the odds of that are slim unless you live in a HUD development. But in general you want to replace items in your home that are older than the typical life expectancies, windows, furnace, water heater, A/C, roof, siding, other appliances, cabinets, counter tops, plumbing fixtures, older than 15-20 years. Carpet, flooring, paint, ceiling texture older than 10 years. Landscaping should be fresh and well maintained. These should all be pointed out to the appraiser.
The biggest things you can do is add finished Sq. footage, finish the basement, add on, add a bathroom, add a gas fireplace, add garage space 3 stalls is ideal, (increase the amenities). As a former home flipper, you want to buy the ugliest house on the neighborhood and turn it into the 2nd-4th best in the neighborhood. Not first best because the other homes bring your value down, but above average so yours will sell fast.
As most realtors will tell you neighborhood need curb appeal. Buyers should want to look at the inside, not be turned off from the street. Additionally I would look at your market area, house prices in general are on the decline, now is the time to make improvements because contractors are looking for work and giving deals. You want to be fairly confident you live in an area where the housing market will recover. Will the job market recover? Is a long commute necessary? Gas prices are bringing buyers back to the city/inner suburbs.