Treat it like a business and buy something that works on paper and not because you like it. Like others have said, be prepared to walk away - once you have done that a few times it makes future purchases a bit easier.
Also, the deal of the century comes by at least once a week.
I'm a spreadsheet kinda gal and when I'm looking for an IP (or even narrowing down an area to invest in) I list all the potentials in a spreadsheet then I have columns that are my criteria ie price, rental return, whether it needs a reno, bedrooms, bathrooms, car spaces, vicinity to amenities etc etc and have a score based on each bit of criteria so a place that is walking distance to a train station would score an 8/10 whereas one driving distance would be a 5/10. I then add up all the scores and shortlist the ones with the highest score to inspect.
Looking at it as raw data on a spreadsheet helps take the emotion out of it.