Be gracious beforehand.
Your parents are probably among your biggest champions. If you complain about his mother to your mom, guess what? She's not going to like her very much. Let the parents form their own opinions and don't start things off on the wrong foot by badmouthing each other's parents to your own.
Plan a simple gathering.
If everyone lives nearby to each other, host a simple get together at your place (or favorite local haunt). Just make sure, that wherever you host, it is quiet enough that you will be able to hear each other easily. If everyone is within driving distance, but not local, pick a place to meet in the middle. This way, one family doesn't have to play host while the others feel like guests, rather than equal participants.
If you are too long distance, start things off with social media.
If the two of you are a long distance from your families, it's probably not feasible to arrange for everyone to meet in person. Some parents might not be as comfortable video chatting as others, so just make a simple introduction on FB so that they can get to know each other before jumping on Skype.
Keep "safe" conversation topics in mind.
Families can be so incredibly different. Avoid talking about religion, politics, and socioeconomic topics. Especially if you already know that both groups of parents will not see eye-to-eye on these issues, have some "safe" conversations in your backpocket, such as how both dads are Cubs fans or how both families love to ski.
Manage your own parents.
The two of you should talk ahead of time about managing your own parents. This really only applies if you expect one parent to be problematic or confrontational. Particularly if you have noticed this parent being less than supportive, make sure you both agree to deal with your own parents at this event.
Make sure to have popular planning questions answered before the meeting.
Of course, the easy focus for this gathering will be your wedding. If you have already started on planning, then be prepared to answer common questions such as the date or where it will be held.
This isn't the time to talk about budget issues or planning disagreements.
And while discussing your flower preferences and favorite cake flavors is totally fine, this is not the setting to figure out if your parents are planning on chipping in or mentioning that you plan on having your stepfather walk you down the aisle.
Design the environment to allow everyone to be their most comfortable.
The goal is for everyone to get to know each other and have a good time. First impressions are important. Make it easier on yourself and make everyone else feel more comfortable by creating an environment that everyone will feel at ease in. For example, if your parents are wealthy, but his aren't, don't host this meal at the country club. And as no one has ever actually felt great in a pair of Spanx, keep it casual.
Openly praise each other's parents.
This is an excellent opportunity for everyone to get credit for the great things they've done for the two of you. If his dad gave you a jersey so that you would fit in at family game days, mention it. If your fiancé received a care package from your mom when he had pneumonia, now is the perfect time to thank her. Gatherings like these can sometimes make parents feel insecure, and being recognized for their thoughtfulness will help them feel more confident.
Pick up the tab.
Do not let your families wage a war over the check. No one wins that battle. If you are dining out, you guys should pick up the bill for this one.