It doesn't get 'harder', but you need to diversify your types of credit more. You can very easily hit 750-775 by simply paying your credit cards on time and with a car loan. However, you won't be able to really get any higher than that unless you have a home mortgage, which of course is also paid on time.
So while it doesn't per se, 'get harder' you have to utilize more types of credit in order to continue to gain. Raising your score from a 686 to a 750 though shouldn't be hard at all. Once you get to a 750 you're pretty well off, and will never have trouble getting credit. Anything higher than a 750 credit score isn't going to help you get credit, you'll just be able to get larger amounts of credit, at lower interest rates.
Keep in mind, this is based off of FICO scoring, which is only available at myFico.Com. Experian, Trans Union and Equifax will try and sell you your credit score at their respective sites, but these are all your FAKO, which have a different scoring algorithm.