Keeping the house clean and fresh is important to get the highest possible rent income.
Take care of the highest quality tenants and don't get lazy. Were you ever in the military? Think along the lines of "signing off" every time you complete a tenant walkthrough.
Would you sign off on the crawlspace or attic today? How about the trees?
Etc. etc. etc...
You might wanna hire an inspector and get a valid opinion with a thorough, visual report.
Finally, follow cost vs. value principles when renovating - check the web reference.
Start by keeping it clean....it doesnt come much cheaper than soap and water. Its surprising how many owners skip over that most vital step....just making sure the outside gets cleaned once or twice a year makes such a drastic difference.