If you're playing a flat-out warmonger game, avoiding denouncements should not be a major concern. You're trying to conquer the world, and nobody is going to thank you for that. If you are playing on a large map and want to form temporary alliances to keep from fighting more enemies at once that you can handle, that can work. Just remember that in the end, there can only be one.
Assuming you want to maintain friendly status with one or two civs for trade, research, or military purposes, here are some helpful things to keep in mind:
1) Prioritize your targets. As soon as you meet a civ, assess their land, what luxuries they have, what strategic resources they possess. Figure out what kind of military threat they pose. Using this information, decide whether you want to conquer them immediately, in the near future, or at some indefinite later time.
2) Do not bother engaging in more than minimal diplomacy with your high-priority targets. Do not sign DoFs with them. Do not give them open borders. If they challenge you to move your troops, refuse and declare war rather than taking a penalty to diplomacy with everyone else when you break your word.
3) Sign DoFs with your lowest priority targets. Denounce the people they denounce, and do everything you can to isolate them diplomatically from others. Bribe them to declare war on your enemies or on neutral civs. Manipulate them so that they believe you (and any other friends you have) are their only allies. Then crush them mercilessly once all other targets are gone (at which point they usually start to suspect your intentions). With a little luck and careful planning, you may be able to whittle down your erstwhile allies by pitting them against each other and siding with one faction, then mopping up the survivors.
4) Never sign defensive pacts. Ever. They force you into wars when you least want or expect them, and tend to ruin your carefully constructed diplomatic blocs. This is true for almost any style of play, not just warmongering.
None of this matters if you don't want or need more than marginal diplomatic contact, of course. If you're not planning on relying upon diplomacy to gain gold, research, luxuries, or allies at any point, then feel free to backstab, betray, and bully the AI. Just know that once you gain a reputation as a backstabber, it never entirely goes away.