To cut to the chase: your trekking poles.
They serve a number of essential purposes. Going uphill, they help you spread the muscle-work across your body, letting your arms and chest help your legs haul your sweaty self up the mountain. Going downhill, they help you control your speed, and keep you from falling on your face as you're climbing down wet rocks.
If you're young and fairly healthy (like me), or cool and fashionable (unlike me), you might think that "trekking poles" aren't for you. They're what your scoutmaster used to use when he was leading you and your patrol around the Grand Tetons. They're what the over-geared nerdy types use, who are afraid of doing any long-term damage to their delicate joints. They are most certainly, however, not for you.
If you've never gone uphill with trekking poles, then you have no idea how fast you are capable of going. Bipedalism is well and good on flat surfaces, but there's a reason that monkeys rule the verticals. Get a pair of trekking poles and you will start floating over the hills like a rugged genie.
As far as going downhill... no matter who you are, you're going to need to come to terms with your mortality. You plus your gear is a lot of weight, and downhill lasts a long time. Without poles you'll find yourself dropping your entire load onto your legs repeatedly for hours, if not days. Unless your skeleton is made of adamantium, that's going to do some damage. Impress your friends some other way.
Packing guides will often list trekking poles as "optional," but they are lying to you. They are as important as a backpack, if not more--after all, backpacks can't really double as lightsabers to aid in the settling of disputes. Details, details.