You can run a reasonably current Debian install by just switching to the testing repos after you install a stable release. This mostly works pretty well but occasionally[1] you'll have an issue. While you could get absolutely up to the minute software (from a Debian standpoint) using the sid (i.e. unstable) repos, I wouldn't recommend it as breakage is quite common there as they are working through various packing issues and that repo lives up to its name.

[1] every couple of years in my experience... typically as they're getting closer to a new release and package breaking changes are needed/slip through.