Something to understand: In Japanese society, the Sempai / Kohai relationship is largely organic. In a status-based society, the senior and junior naturally recognize their obligations to each other, and follow these social norms without issue.
In Japan, the Sempai / Kohai relationship is not simply a one-way relationship. It's not simply the junior having an opportunity to train with a senior member, it's not about simply communal respect... Any attempt to simplify the understanding of the relationship is to neglect its sociological importance in an upwardly motivated society.
Sempai (先輩) – The Elder/Senior
Most westerners attempt to understand this relationship in the terms of the Master/Protégé relationships of artisans and their assistants. This is incorrect. The sempai is a person acknowledged as having earned the privilege of a higher status by means of seniority within the organization. Age or even ability do not necessarily dictate sempai status.
Within martial arts, typically this sort of role is fostered by example; the advanced students (dan and high kyu ranks) are encouraged or placed with students that are newer to the organization. They recall being taught in a manner that their senior would instruct them in fine points, to correct them in issues that the sensei might miss as he watches over the whole group. The sempai, therefore, is responsible for the growth of the kohai. In this way, the sempai is responsible for the guidance, protection, and teaching of the kohai.
In businesses, you might view the manager or supervisor as sempai, and this would largely be accurate. It's important to note that this relationship better illustrates the importance of the sempai role – in fact, his advancement and development is as tied into his kohai's abilities as his own. A poor manager does not get promoted.
Kohai (後輩) – The Junior
The kohai has much expected of him as well. In many schools, the kohai is often burdened with the "grunt" work, expected to clean the dojo, stack mats after training, and may even be required to do laundry. This is often viewed as a means of testing the resolve of the kohai. Essentially, the expectation of the kohai is to respect, obey, and assist their sempai, and this relationship will generally continue for as long as the two maintain contact, regardless of any transition in status.
Benefits of the Sempai/Kohai relationship
The relationship between these two roles is symbiotic; both parties are benefitting from their interaction. The sempai gains the experience necessary to become sensei; kohai gains the experience necessary to become sempai. If both parties acknowledge and respect their part in the society, then the formal nature of the class is upheld, and a natural order is maintained in the class as a whole. The sensei / sempai / kohai hierarchy is symbolic of the larger hierarchy of feudal structures which defined and shaped Japan as a society, and continue to do so even in the post-Imperialist age.
The individuals benefit from this sort of relationship on an individual level; it's vital that the newly indoctrinated be protected and trained, that the elder students learn what it means to lead and to follow (you are, even as sempai, still kohai to someone) and to be responsible not just for yourself but for the class as a whole.
Fostering a natural sensei/sempai/kohai hierarchy in the dojo
[NB: I found this structure common in a study of cults I did at university. I've adapted it to the use of Japanese names, but this pattern exists in cults, secret societies, religions, pyramid schemes, and even in a great number of highly commercialized dojo. TAKE NOTE: This method is extremely subversive and not recommended. It is posed here as one way of establishing a hierarchical microcosmic society.]
Introducing this as a concept is artificial; the students should never be expected to adopt an entirely new way of acting and behaving and be expected to not rebel. Instead, an indoctrination format can be adopted to create natural hierarchies within the classroom, but it must be done patiently with a longterm goal in mind.
Create a hierarchical divion. When you examine your class, find the median rank. Sometimes it's easiest to divide at around 4th kyu, but this may alter based on your class size and ranking. A scatter-plot of your students ranks can help with this. The "upper" group will be sempai, having earned more rank. The lower division will be kohai.
Match your own pairings. When you break apart for waza, tell ranks x-kyu and up to pair with a junior student. Start by using the terms "senior" and "junior". You're cementing an idea of rank being an important issue here insofar as who can train with whom. After a couple of months, start slipping up once in awhile: "Seniors, join up with your kohai, er, juniors"... Then after a while stop correcting yourself... Sometimes you sempai for seniors, other times used kohai for juniors. After a while you shouldn't need to correct or use english terms at all for these two divisions.
Reward natural pairings. When a senior student naturally pairs up with a junior student, acknowledge the senior student. The junior student should want to pair with the senior student, as they should understand they stand to learn more, and thus need no acknowledgment. In fact, they'll seek it more if they never seem to receive that attention.
Address the senior students. When you teach, teach to the sempai, and the sempai will teach to the kohai.
Maintain and enforce hierarchy. Traditionally, students are lined up according to rank, usually front to back, right to left, highest to lowest for bowing. This may differ based on the organization, but insist on the order. You're affirming the importance of hierarchy.
Do not explain yourself. You are sensei. You do not need to explain yourself to your subordinates; they are all kohai to you, and their place is to obey and respect you. The fact that you want things this way means that should be enough of an explanation. Encourage those who follow, do not waste much time with those who don't.
I'm not encouraging you to take this route, but this is a way that you can indoctrinate (and I use this term because it's appropriate and reflects the sort of attention this behavior draws in the west) your students into hierarchical structures. Make the change gradual, and they won't notice anything amiss; too rapid of a change and everything will fall apart.