We often see how being too assertive or gung ho can be hazardous, however, so can being too gun shy or passive. Failing to define and communicate clear boundaries can get us (and others) into trouble as easily as being hot headed. The first step of this process is being clear on what your boundaries are. This may sound easy, but it's not always as clear cut as one might think. Take something "simple" like losing weight. Going on a diet to limit your calories (setting a boundary) sounds easy enough, however people fail at it because it is more difficult than just eating less. Being clear about what and why you are doing something is necessary. The clearer you are able to connect your why, what and how (and leave some room for a little flexibility) the more successful you'll be. It is easy to drift off target, so clarity is important. The clearer the better.
The how involves communicating your clarity to others in a way that gets your point across non-verbally, verbally and of course with your actions. Communicating boundaries consistently over time will build a habit. That habit can often carry you through times when your resolve falters. Also, the better your communication skills are, the more effective you will be able to outline the parameters of your request to others. If you have difficulty communicating your boundaries, it's only a matter of time until they are challenged. Heck, they will be challenged even if you do communicate them well, let alone if you communicate them poorly or not at all.
Lastly, have a plan "B" in place so you know what you are going to do if and when your boundaries are not respected.
This entire process can be difficult for obvious reasons, not to mention there is usually incentives for us to not see or communicate our boundaries. These incentives may be simply keeping inside of our gun shy comfort zones, or they may be because of other tantalizing distractions, such as some tasty cake to fill our dieting bellies. The short term satisfaction will eventually give way to longer term regret, drama, and sometimes even dangerous side effects of not holding one's boundaries.
It gets easier once you get used to it. Start with small things and train a habit of clarifying and communicating boundaries that are most good / least harm for everyone. And as always... Keep going!
All the best,