I hate running with anything in my hand (mostly because my arm gets tired and it reminds me how weak my upper body is). But buying the right handheld bottle can make all the difference on the annoyance scale. Look for a bottle with a wide cinching strap, like the Nathan bottle on the left. Try it on, pull it tight, and violently shake your hand around. Does it come loose or fly off, narrowly missing someone’s head? If so, you probably grabbed a bottle with a narrow strap, like the one on the right.
The less energy you have to put into holding onto your bottle or re-tightening the strap, the more energy you can channel into trashing your quads on that next downhill.
Bonus: the bottle will protect your hand if you ever dive headfirst into the trail. I may or may not have firsthand experience with this.