1. Equip a small, single-handed item (such as a battery).
2. Place your arms at your sides, i.e. not in combat stance.
3. Begin sprinting by double-tapping W.
4. Hold G, let go of W.

For as long as you hold G, you will continue to run at max speeds, even up-hill.

Doing so up flights of stairs (i.e. in buildings) or certain types of terrain will immediately kill you.

Checked as private to avoid spreading awareness. It's easy to duplicate.