What to think about before you choose
It sounds obvious, but many people forget to consider what they are interested in. Many others make the opposite mistake, and only consider what sounds interesting.
Of course, what sounds interesting might be very different from what is actually interesting: if you're really interested in climate change, for example, environmental economics might sound like a good option - unless your environmental economics module barely touches on climate change and instead focuses on the sustainability of forests. You need to look further than just the title (see the ‘sources of information' section below.)
2) Quality of lecturer/teacher - crucial
Although efforts to improve the quality of teaching across the university system are being made, it remains the case that some lecturers are better than others.
The importance of choosing a good teacher cannot be overstated. Greg Mankiw, of Harvard University (and author of a widely used macro textbook) writes on his blog:
‘Choose your economics courses from professors who are passionate about the field and care about teaching ... All parts of economics can be made interesting, or deadly dull, depending on the instructor'
Actually, he goes further, claiming that the content of each course doesn't matter. This is perhaps a little strong, but there is no doubt that good teaching can go a long way in keeping you interested.
3) What you want to do next
Something not necessarily stressed to students. In immediate terms, not doing a certain optional module in one year might prevent you from doing another, related, module in your next year. Alternatively, not doing a certain option may make you less desirable as either a future employee or a future graduate student.
The only way to check the former is to look at the optional modules available throughout your course - and to ask if it is not clear what any prerequisites are.
Fortunately, most employers care about your overall performance rather than which modules you have chosen. But this doesn't always apply. In particular, if you are interested in pursuing economics beyond your undergraduate degree, specific modules may be more desirable. See ‘Where Next?'.
While different optional modules cover different areas, there often is a degree of overlap between them, and between optional and compulsory modules. Spillovers can come in two types:
Skill spillovers - a crude way of thinking about economics is that it is split into essay-based and more problem (maths) based areas. If you only do one ‘problem' module, it is likely that you won't be as sharp at maths as someone who does several. Conversely, the same is true of writing skills and essay modules. As always, balance is important.
Knowledge spillovers - many optional modules touch on similar areas to other optional modules or compulsory modules. While the approach may be slightly different in each case, being able to draw on a body of related knowledge that most students won't will normally impress an examiner.
It's worth bearing in mind that spillovers can work between years, as well. Although a final year module might have no prerequisites, it might help to have done a particular penultimate year module (see above).