Google Analytics is a great tool. We use it at Hosting.com and it's something that I look at everyday. However like any tool, it isn't perfect and there are stats that you shouldn't necessarily take for granted. One of those stats, and it is an important one, is your conversion rate.
I for one have never paid much attention to the conversion rate that Google Analytics reports. I instead prefer to calculate this number myself with actual sales from our billing system. I would recommend that you do the same.
If you're going to use Google Analytics, or any website stats program for that matter to calculate your conversion rate, I would take a close look at your traffic sources. Some months you're going to purchase online advertisements and they're just not going to work. That's just the nature of marketing; trial and error.
Those visitors probably aren't sticking around for very long on your site and the bounce rate is probably quite high from those traffic sources. That's obviously going to negatively impact your conversion rate. Don't be afraid to strip those traffic stats out to give yourself a more accurate idea of what your conversion rate was for that month. Also feel free to remove stats from any pages on your site that aren't necessarily going to convert users. An example is a blog post that is getting a ton of hits, but just isn't converting your visitors. You can easily accomplish this by using Analytics to filter and segment these pages and traffic sources.
Stripping out these numbers from your traffic stats to calculate your conversion rate will allow you to more easily compare your conversion rate month over month and year over year. After all, your conversion rate is a key statistic to help you measure and really compare the effectiveness of your marketing efforts.
