Most of the time, when we're discussing D/S relationships on the boards here, we are talking about real life relationships; people who if they don't live together at least are meeting face-to-face. But just from an anecdotal perspective, I think far more of the D/S relationships that most of the members here are engaged in are largely strictly online. For the most part, the guidelines that you follow for those relationships should be identical to those you follow with real-life relationships but there are some important differences.
The first and foremost concern should be the safety of the submissive, just as in offline relationships. Ladies, if you are doing something and it feels extremely painful, if there is blood or something just feels wrong, stop what you're doing immediately. Experienced doms aren't going to ask you to do anything overly dangerous but they will also immediately stop everything if you are concerned about your safety and health. If your own dom isn't doing that, stop anyway and you should probably reconsider whether or not to continue in your online relationship with this dom. Chances are that unless one of you is a medical professional, that if something feels excessively painful or is bleeding you aren't going to know if she is going to need medical attention so err on the side of caution in those cases..
A thing to remember also is that the power in the dom/sub relationship always should rest in the hands of the sub. She (or he) has the magic word at their disposal at any given moment: no. Wield your magic word with impunity and don't feel that it makes you any less of a sub. You have the right to regulate what's being done to your body; use that right as you see fit. If you're okay with shoving a cucumber up your poo-hole, go for it but if it comes out with part of your intestine attached, don't wait for your dom's okay to stop what you're doing. Safety always always ALWAYS comes ahead of your dom's pleasure. Without exception.
Real life also comes first. In many cases here, online dom-sub relationships include people who are in offline relationships. Those relationships come first. Doms should realize that they do not have claim to their sub in anything but an online manner. Their subs have obligations to those they are with and often, the doms do too. Subs should understand that no matter how much they need their Doms, his/her real life relationship comes ahead of theirs. If you can't handle this, you shouldn't get involved in an online D/S relationship. Careers also come first so asking your sub to show up at work in a dog collar and sheer body stocking is not going to happen unless your sub works in a fetish parlor.
Like in offline D/S relationships, boundaries should be clearly understood by both parties; if your sub is triggered by anal play and she says so, stay away from that hole. You don't have the right to trigger your sub no matter how badass you might be. Trust me, all you prove by doing shit like that is that you're a shitty dom and a shittier human being. Good doms understand what their subs limits are. Pushing them can be okay, but there are situations where it isn't. Learn the difference.
Expectations should also be discussed and both parties should understand what their partner is looking for, what they expect and what is expected of them. If something comes up that hasn't been covered in conversation yet, don't make a unilateral decision - discuss it with your sub or your dom where appropriate and make sure that you're not crossing a line.
Some doms like to get exclusivity with their subs; they don't want their playmates playing with other people without their express consent. Whether that is agreeable to the sub in question is up to them and there are some subs who crave that exclusivity but not all subs do; after all, most of us are here to play and if they are restricted in who they can play with, they may not want to be your sub for very long. Again, there's that magic word - no. If this is a deal-killer for you, again you might want to find yourself a partner who is more suitable for your needs.
Because of the nature of online relationships, it is easier to lose yourself in the fantasy than it is offline when you have to put up with all the realities of an offline relationship. Keep in mind that online relationships are generally fantasy-based; you are seeing the person on their best behavior and they are projecting an image, one they want you to see. Sometimes, they don't want you to see the real them because for them, it spoils the fantasy. Online relationships are very much like the first weeks of being in love; it's heady and fun. However, be wary - you never know who you're really dealing with; your dom could be a sadistic rapist who wants to get you into that real life meet so he can hack you into pieces with a machete. Your sub might well be a serial stalker who gets off on ruining the lives of doms. Take steps to protect yourself, and be suspicious of online doms or subs who are overly insistent on real life meetings.
We sometimes use the term "insta-doms" around here to denote men who think that because a woman is submissive, that means she is submissive for every dominant. That is simply NOT the case and the sooner you realize it the better. A dom/sub relationship is like any other romantic/sexual relationship; the submissive chooses whom she gives her submission to. You can demand it all you like; chances are if you're too strident about it she will tell you to fuck off - and you'll deserve it, too. Be respectful of your sub and perhaps she'll give you the respect you're looking for. Again, the power in any dom/sub relationship rests with the sub. Good doms know that.
It's very intoxicating to have beautiful sexy women or handsome sexy man willing to do whatever they're told to do for you; there is also quite a thrill in giving control to someone you trust. There's nothing quite like either of those roles, and anyone who undertakes a dom/sub relationship, even online, I wish the very best. Like any other relationship, there should be mutual respect between both parties and each should treat the other with empathy; it is advisable that if you are looking to get into D/S relationships that you spend some time as your opposite - doms should be subs for someone else just as subs should be dominants at least for a week or so just so you get an idea of what's involved. There are also lots of veteran doms and subs here in RC who will be happy to answer questions if you have any; there are plenty of threads on this board that are also useful, along with links to other sites that contain even more information. Take advantage of them. Above all, play safely and if you are concerned that your sub is going overboard, or that your Dom is not keeping your safety in mind, communicate your issues to them and if necessary, end the relationship. No online fun is worth getting someone injured for.
There are lots of pointers I'm sure I've forgotten and I invite you to add tips and comments below.