Appraisers look at the home's construction quality and exterior features. Construction quality is detailed in the Marshall & Swift Residential Cost Handbook. Obviously, a custom quality home is more valuable than a residential tract home. Other exterior features noted include: quality and condition of landscaping, paint, gutters/downspouts, windows (type and condition), roof and exterior doors.
Interior features observed include type and condition of floor coverings, heating and air conditioning systems, updating and remodeling, any additions to the original structure, overall maintenance level, obvious repairs needed or other deferred maintenance items like holes in walls, peeling paint, water damaged ceilings, leaking faucets and evidence of dry rot or mold.
Look at the exterior and interior of your home as if you're seeing it for the first time. Minimize the defects and accentuate the positive. Make whatever improvements can reasonably be done within your timeframe and budget. Understand that the appraiser is looking mainly at structural components of your home, but also reports the overall appearance and maintenance level to the lender.
Appraisers generally do not care about housekeeping issues like dishes in the sink, laundry that needs to be folded or beds that are unmade. These types of things are irrelevant to an appraiser.
If you have a pet that might be considered a nuisance, make sure it does not come in contact with the appraiser. Appraisers inspect multiple homes in a given day. They don't want to have to tell Sparky to "get down" 40 times. Also, make sure any animal droppings are picked up in the yard. There's nothing worse than the appraiser having to negotiate "land mines" while performing an inspection.