Traditional maritime navigation can be helpful on sunny days in Manhattan.

1) Look at your time-keeping device to determine morning or afternoon (if it's not obvious)
2) Try to find the Sun's position in the sky (you should at least be able to find its light reflecting off a building)
Once you've oriented yourself in direct position to the Sun you will be ready to navigate.

If the Sun is over your left shoulder in the AM, you're headed Downtown (South) on an Avenue; left in the PM you're you're headed Uptown (North) on an Avenue. At noontime, with the Sun directly towards you, you will be heading Downtown.

These rules only apply to Manhattan North of Canal Street, as Downtown Manhattan's Avenues do not neccesarily parallel the Earth's lines of longitude, and cannot be used in The West Village due to the same issues.