I think arranging a marriage for the sake of peace alone is a bad idea. What are they going to do force Rosaline to abandon her chaste ways to marry Benvolio? There may be others out there, but a marriage such as this would likely be an unhappy one and likely bring even more animosity between the two families.
The raising of statues in each other's honor is a good start. Beyond that both Lord Capulet and Montague must meet with his family and servants and tell them no more fighting will be tolerated. The message must come from the top and it must come with the understanding that breaking this wish will come with the highest of consequences. I believe the needless bloodshed will, in retrospect, make most members from each side realize the futility of the feud. As poster #7 said, without Tybalt around, cooler heads will likely prevail.
Not to say that Tybalt would, in himself, declare death to the peace at the end of the play, ... but I have to say that having him dead is certainly a start.
What, drawn and talk of peace? I hate the word, as I hate hell, all Montagues, and thee.
Tybalt, being the epitome of the generations of hate passed down, has died through his own hatred. Step one achieved.
Step two would have to be choosing an impartial mediator between the two sides. This has already been done at the end of the play: the Prince makes a wonderful mediator. Keep it up, good Prince!
Step three, I would think (and keep in mind that I am thinking of this through my modern mind) would be to get the groups together in one place. Peace talks, so-to-speak. Any Montagues and Capulets found to have true, lingering animosities towards each other should be counseled, ... and closely watched. (Perhaps the Friar could help?) I think through these talks, they would find that much of their reasonings died generations ago, ... much like the feud between the Grangerfords and the Shepherdsons of The Adventures of Huckleberry Finn.
Step four, would have to be what has already been suggested: to encourage marriage and friendship between the two groups. Step four without steps two and three could definitely end up in regression to the feud.
Therefore, just perhaps, we could end up with some true Montague/Cauplet happy-happy, joy-joy. Or, ... maybe not. Ha!
We'd like to think the two families--along with a town which fed into their feud--have learned their lessons and will move forward in peace. However, that in not particularly likely since blood has been shed. Talking with the Prince obviously did no good the last time, but perhaps he will have a little more influence now. The families erecting monuments to the young people they lost is as good a start as any, for the statues will serve as reminders of loss because of the senseless family fighting.
I think that the key would be to look to the past animosities, forgive and encourage unity between the two families. Actions such as building a monument to the young lovers, maybe even somewhere that both families could share as a meeting place, place of worship or place of contemplation could help to promote an appreciation and understanding between the Montagues and the Capulets.
In accord with the idea of the two families merging through marriage, this is probably the most practical idea. Animosities are sure to surface again, so if there are couples who are married, the other family members may be more temperate about their animosity.
Another alternative would be the transition of one of the families to another city. For, moving away solves many problems.
In the time period in which the play is set, the best way to promote peace would probably be to do just what Romeo and Juliet were trying to do -- have a Capulet and a Montague marry. Back in those days, marriages were arranged all the time for reasons like this.
If the two families are truly serious about learning from their mistakes, they should try to join together. A really effective way to do this is to have members of the respective families marry. We do not know for sure if there are available family members who could marry, but I think that would be the surest way to promote peace.
This is a real interesting question that is raised in extrapolating the ending of Shakespeare's play. Naturally, he leaves it unanswered so that we can discuss it in forums like this one. It is Shakespeare's genius to be able to give such a transcendent ending and allow generations to speak about its meanings or applications.
The first step would be to take what is offered in the text. Capulet offers his hand to Montague, over the corpses of their children. Both promise to raise statues or monuments to the young lovers. The Prince concludes the play suggesting no tale sadder than that of both the children. From this, reconciliation is said to follow. I think that the unification between both houses has to be overseen with a third party. For most of the play, the Prince is fairly ineffective, always arriving late to the problems and short on solutions. I would think that this ending gives the Prince a significant moment to demonstrate his effectiveness. The Prince can articulate rules and guidelines for both families to follow. We really do not get a feel as to why there is animosity between them, as we only know there is. They are "bitter enemies for centuries," but we do not know the origin of this animosity. There would have to be an addressing of the fundamental chasm between both families and assurance that negotiations replace violence. At the same time, the forces that are loyal to both families would also have to move away from armed conflict as a solution and use the power of mediation, where again the Prince can be effective.