Everyone has boundaries and limits in their life and relationships. They can be physical (such as not allowing others to drive your car) or emotional (such as using words like "partner" instead of "boyfriend" and "girlfriend").
Recognizing, communicating, and enforcing boundaries with your partners is essential to maintaining a healthy relationship. Let's address each individually.
First, you must recognize your boundaries, and determine how firm they are. Some will be non-negotiable. Some will be flexible under certain conditions. It's important to make this distinction.
For example, one of my boundaries involves cohabitation: I do not live with partners. This, for me, is inflexible. I am very solitary and greatly value my living space, and it's vital to me that I be the only one who sets the rules about what goes on in my home. This includes everything from who is invited over to how much clutter is present.
When I first started dating Rusty and Ryder, I had an additional boundary of no sleepovers. This stemmed from a previous, abusive relationship which had left me feeling highly vulnerable. After I worked through some of those issues on my own and established trust with my partners, we renegotiated this boundary and it no longer exists.
So keep in mind that boundaries are fluid, both yours and your partners'. Just because it was once true doesn't mean it must be true now, and it's okay to let your partner know that something has changed from what you had previously determined.
Which brings us to the next point: communicate your boundaries. Don't expect your partners to guess at what's important to you. Trust me, they'll guess wrong. Tell them what you need, and what is too much. Don't be afraid of hurting their feelings; it will hurt them far worse to do something they thought was okay and unintentionally make you unhappy.
When communicating your boundaries, be clear and be firm. Don't hint at what you mean, especially for issues involving sex. Some people find talking about sex uncomfortable, but the people you have sex with need to know what you consider out of bounds.
So if what you mean is, "I don't like anal sex and don't want to have it," say that, don't hint or insinuate. Leave no room for doubt about what is important to you.
Even if your boundaries are fuzzy, communicate that clearly as well. It's okay to say, "I'm not sure how I feel about anal sex. I haven't really tried it. I might not like it." If there's information you would like from your partner, ask for it. If there are things that bother you but you're not sure why, let them know. The more information your partner has, the easier it will be for them to do what you need. Also, having a discussion with your partner when you're not really sure where your boundaries lie can actually help you figure out what is important to you and why. Sometimes just talking about it is enough to give you clarity.
Finally, enforce your boundaries once they've been defined and communicated. If you've told your partner you're not okay with sleepovers and they fall asleep on your couch, wake them up and ask them politely to leave. Any time your boundaries have been violated, explain to your partner what the violation was, remind them of the conversation you had in which you communicated what was and wasn't okay, and insist that they maintain your boundaries in the future.
By the same token, respect your partners' boundaries completely. You can, of course, ask the reasons behind what they ask of you, but be prepared for the answer to be "I don't know, I'm just not really comfortable with it." Check in with them regularly if you feel a boundary might be renegotiated, or is sensitive enough you feel it needs extra care. If you find yourself unsure of whether you're violating your partner's boundaries, ask, "Is this okay? Could I be doing something better?"
Don't be afraid to communicate your boundaries, and don't be offended when a partner communicates theirs. "I'm not comfortable having unprotected sex" doesn't mean "I don't like you."
Respecting the limits your partner has regarding themselves, their property, and their behavior within relationships is a basic element in a healthy relationship, as is having your own limits respected. Remember, you have the right to protect your body, your money, your home, and your safety. You do not have the right to violate that which belongs to your partner. If you share some of these things (like a home and finances), ensure you have clearly established the boundaries about what is and isn't okay to do with them.