Lifting a Kayak Safely - Use Your Legs or Get a Buddy
Kayaks weigh a great deal more these days than when I started paddling. Back then, kayaks were made from fiberglass and weighed between 20 - 25 lbs. Today, nearly all kayaks are made from roto-molded plastic and some weigh up to 50 lbs. After you finish a paddling trip, you may have some water remaining in the boat. Water is surprisingly heavy at 8 lbs per gallon. I suggest getting in the habit of draining your boat before carrying any distance.
Be honest with yourself; this is no place to be proud. If you are getting up in the years and need help, ask for it. This goes especially for those with back injuries. We boaters are a very friendly lot for the most part and many will quickly help you carry your boat. Another place it makes sense to ask for help is portaging over rough terrain, or after a really bad beat down. Using two people is not only much safer, it also saves a great deal of time as well. If you are young and full of energy, keep an eye out for those that need help and offer your assistance. Many are afraid to ask but will gladly accept assistance when offered.
Here is a nice article with videos on how to properly lift a heavy object like a kayak and avoid back injuries: How to lift heavy objects. This is a nice article from an Orthopedic doctor on lifting heavy objects and preventing back injury: A Doctors Take.
Carrying the Kayak
OK now that we know how to lift a kayak properly, it is time to carry it to the waters edge. Kayaks these days weight around 40 pounds. Add a little bit of gear and we are right around 50 pounds. Most kayakers balance the boat on their shoulder. If you have a sponge and it is a long carry, it isn't a bad idea to place the sponge under your shoulder strap as padding. When you arrive, bend your knees to lower the boat closer to the ground and ease off your shoulder.
If you encounter rough or slippery terrain, don't risk a fall - it isn't worth it. Personally, I drag the boat on the ground using the handle. If you need to go down a steep embankment, you can set up an assembly line or lower with your throw rope. The same rope trick works well going back up hill.
If elderly, have back problems (any age), or other medical issues like heart disease - ask for help. Boaters are a very friendly bunch, at least the ones I paddle with. Why take chances on an injury or worse?