A significant point for any small business is that, if you don't appear to be ready to fight your corner, you are less likely to be treated seriously and are more likely to come off worse, particularly financially.
That doesn't mean you have to spend a fortune; it does mean that you have to look sufficiently stubborn and determined - to look more like a fox than a chicken or a rhinoceros than a mouse.
We like to stand up for SME's and stop them being walked over - and I have successfully acted for small firms against large companies like Honda, Mattel, Mercedes, Mars, and Neill Tools, often with large city law firms on the other side.
Of course our services don't come free, but our fees are reasonable (and start at £500), and can be cheaper than having your goodwill damaged or losing sales -and having a reputation for not being a pushover is worth more in the long run I think.
It's important too that your case is argued to the best effect - if the other side think they can bully you or ignore you, or that you don't understand the legal issues properly, you're unlikely to get the best outcome.
In short, the psychology involved can be just as important as the legal rights and wrongs.
Yes, if a case looks as if it might need to get to court, it can become very expensive, but we also have close contacts with a large firm of solicitors who may be prepared to take on appropriate cases sometimes on a no-win-no-fee basis.
It isn't essential necessarily to always challenge an alleged infringement as soon as you are aware, but you shouldn't leave it too long - the longer you leave it the harder and more expensive it gets generally.
Also, if you leave it too long, as you say, you can be seen as having accepted it if the other side can show that you were aware of their use.
The worst thing you can possibly do though is to take action and then simply give up at the first hurdle - that certainly amounts to consent and is worse than doing nothing. Sometimes, you need to nag away at people until they are prepared to compromise - it's a little like debt collecting - and we don't get paid for being nice to the other side!
The main thing you should do, essentially, is have a strategy, decide your fall-back and where you can compromise, and stick to it - and that doen't mean to say going to court regardless, occasionally infringers can be turned into licensees for example.
The main thing, of course, if you want to protect yourself is to apply for trade mark registration, that's the basic first step.
I hope this helps.