I just discovered Duncan's post on Reneging Prosocially and I appreciated the work it did putting a focus on predictability, transparency, and care for others. It inspired me to write up my thoughts on a closely related subject: How to avoid needing to renege on commitments.
These are strategies I've developed over a lifetime of having memory and motivational issues, and trying to limit their impact on other people. They're not a substitute for using skills to make sure you do what you say you will, but they're a valuable way to deal with lapses. They may also be useful to people who don't have trouble upholding their commitments, but sometimes find that their commitments become costlier or less valuable than they'd intended.
This is the easiest way to stay out of trouble! Instead of promising to do something every week (implied: every week forever), set an initial trial period (eg, every week for a couple of months) and let the other person know you'll evaluate continuing when that gets closer. If your counterpart is the kind of person who likes explicit expectation-setting, you can even make a calendar appointment for the end of that period to discuss how it went and let them know if/how you want to continue.
Set realistic, shared expectations about how reliable you plan to be. For example, last time I was traveling I told my partner that I'd plan to talk to her on chat every night, but there would probably be a few days when something was happening at night and I wouldn't be able to. When that happened, it was no big deal because she only experienced the trivial pain of not getting to talk instead of the greater pain of having her partner let her down. (If I'd been unavailable every night, or not committed to trying, that would have been a different problem -- but not one having to do with breaking my word.)
This often involves promising to use one of the prosocial reneging strategies. For example, if I promise to provide space for my friend's weekly meetup, I can tell them "I'll block this out on my calendar, but I can't promise I won't _ever_ have conflicts. If something comes up, I'll let you know the moment I find out and I'll help you find another location if I can." I like this approach because it brings lapses into the system, instead of making them a violation of the system. Just make darn sure you can keep the secondary commitment!
In general, this is a risk-averse strategy. It trades the potential reputational or relationship damage of messing up for the definite damage of telling people you're the kind of person who sometimes messes up. For someone like me, who's likely to mess up at some point, it's a good deal! If you're a highly reliable person, you might want to use these strategies much more rarely.
Appropriateness will vary with culture. Using these strategies could bring reputational benefits if you're in a social group that values forethought and accuracy, and they could make others feel respected and cared for in a group that routinely acknowledges human biases and foibles. On the other hand, there are groups where unconditional, open-ended commitments are a strong norm and doing otherwise will make you sound weird, cold, or uncommitted.
Sometimes an unconditional commitment is an important social signal or a way of offering someone else a sense of safety. Acknowledging that you might screw up could be seen as a sign you don't care enough. It's similar to the way most people don't have a section of their wedding ceremony about the circumstances under which they might get divorced! On the other hand, this kind of situation also makes a breach of your word much worse, so it's more valuable to plan ahead for it. In some cases it might help to 1) reaffirm how much you care about the person and 2) make sure that any recompense you promise is big enough that it would actually make them feel better and repair their trust.
Using these strategies might reduce the power of the commitment to motivate you. You might not try as hard to avoid lapses if they feel like they're an expected thing you can make up for transactionally, rather than a real violation (like in the famous Israeli daycare fines study). If commitments to others are an important way you get yourself to do things, you might want to lean extra hard on other motivational / habit mechanisms, or just consciously remind yourself why doing the thing is important to you.