It certainly helps to have reciprocal agreements with other carriers as far as jumpseating goes, but I have had luck myself without them. I fly for a non scheduled 135 freight company which makes it difficult to gain agreements with other airlines, but there are a few companies out there who have open jumpseat policies. My company has a handfull of agreements with airlines, but these certain airlines usually operate in only 1 or 2 regions in the country. I can think of at least 3 majors I have jumpseated on recently and in the past, and I have had no problems whatsoever. You need to go to the gate and fill out a jumpseat form, and be prepared to show your company id, pilot certificates, and even your medical. Depending on which airline you jumpseat with, the captain will either sign your form at the gate, or you will have he/she look at it on the airplane. Whether or not the captain signs the form at the gate, you should still go into the cockpit, shake hands and introduce yourself. Be professional and courteous at all times. Dress in your pilot uniform or a business suit with a tie. If you are offered a meal, turn it down unless the crew insists, and by all means do not have a beer or cocktail. I hope this may help you out. Enjoy the free travel.
Best of luck