How do you decide whether a project is worth continuing or not?

It seems the biggest obstacle for many marketers is they become so emotionally involved in a project that they ignore signs of it not being commercially viable. This is why it is so essential to track every action in the marketing process; ie advertising media demographics, ad/promotion response rates, email opt-in rates, open rates, click through ratios, EPC, etc.

Set goals and target measurable objectives to reach those goals. For example, as an affiliate marketer I've made many thousands of mistakes over the years in product choices. But what I would do for any product that didn't sell at a predetermined level over time (widely variable depending on the niche) was to replace it with another niche-relevant product. Sales and other metrics were closely monitored, and anything that didn't perform within pre-established criteria was changed.

Don't interpret this to mean to give up on a project or product when the going gets tough, but you need to develop a systematic way to determine the root of the problem. Even major companies have made colossal marketing mistakes after spending millions on "market research". Sometimes it can take more discipline to pull out of a failing pet project than to continue slogging through it. The difference between goal-oriented determination and lofty stubbornness is that the former knows when it's time to try something else or move on.