Slate roofing tiles are very delicate and can easily crack and dislodge under your weight. Ideally, you would only allow specially trained roofers to work on roofs covered in slate, but for emergency repairs or the retrieval of the occasional frisbee, it can't be avoided. If you must go up there, be sure to follow these rules so as to evenly distribute your body weight.
Choose a ladder that is relatively flat along one side and that is long enough to stretch from the eave of your roof to the area on the roof that you need to reach. Begin wrapping this ladder's upright beams, not its rungs, with old rags and towels. Secure the rags and towels into place as you work by fastening them with bungee cords or nylon zip ties. Continue wrapping and tying until the flat sides of both uprights have been completely protected with towels and rags.
Set up another ladder next to the eave of the roof. Be sure to position this ladder as close as possible to the area of the slate roof that you need to reach, and try to make sure that there are no permanent obstructions in the way like chimneys or vents. If you find it necessary, climb to the top of the ladder and have your friend hold the upright beams to evaluate the ladder's steadiness.
Pick up the padded ladder with the help of your friend and carry it to the ladder that is set up. Use one hand to hold the ladder above your shoulder, making sure that the padded side is facing down towards the ground. Use the other hand to hold the upright ladder's rungs for balance. Instruct your friend to hold the other end of the padded ladder and to help you support it as you hoist it onto the roof.
Climb the ladder slowly and carefully until your shoulders are above the eave of the roof. Steady yourself so that you can use both of your hands to move the padded ladder at the same time. Slowly push the ladder, padded side down, over your shoulder and onto the roof. Instruct your helper to stay as close to the upright ladder as possible, supporting the padded ladder as well as possible.
Slide the padded ladder up the roof, being careful not to scrape or pry up the shingles or panels (you'll be pushing the ladder against the "grain"). The ladder is in a good place when it provides a straight path from the eave of the roof to the area that you need to reach.
Retrieve any tools that you may need while on the roof, then climb the upright ladder again. Carefully climb onto the padded ladder, which should be laying flat on the roof. Carefully advance along the ladder until you reach the part of the roof that needs work.
Climb backwards very carefully when you are finished working on the roof. Instruct your helper to get behind you again as you carefully pull the padded ladder back down. Once the padded ladder is off the roof, you can remove all of the padding and put all of your equipment away.
If you can help it, you should always leave this job up to the professionals. But that isn't to imply that all roofers are experts on slate roofs; many roofing companies don't teach their workers about the delicate nature of these harder-to-find roofs, which is an accident waiting to happen. Search around for a roofing company that advertises its proficiency with slate roofs. These roofers know that experience with slate roofs is a strong selling point, and they will usually tell you upfront that they know what they're doing.
This method can only be used on roofs that are only slightly sloped. If the roof is steep, it will not be safe to lay the ladder on top or to climb on that ladder. Remember that the ladder laying on the roof is not secured by anything but gravity. Use your common sense and don't take any unnecessary risks.