It can be difficult to track the exact position of keywords in Google since a number of things can affect the outcome. Personalized search, local results, and different data centers can all dramatically influence the rankings you see. Google's algorithm changes more than 200 times a year, so rankings also fluctuate from week to week or day to day. Overall, you should be looking for trends in your rankings.
With that in mind there are numerous tools that can help you track rankings. One way to track rankings is to use filters within Google Analytics. More information about this is available on Yoast's blog.
This information can also be gathered through Google Webmaster Tools. In your Webmaster tools click on "Your Site on the Web" and then on "Search Queries." This will give you the average position on Google for your site for a given term.
Paid options also exist that allow you to track very specific terms. Rank Tracker is a paid option that automatically checks your search engine rankings and shows you if your site has moved up or down in the search results. It can also help you discover profitable keywords you may not have considered on your own.
The SEO Moz tool is another good option that will track rankings from Google, Bing and Yahoo in any country. This tool pulls rankings automatically and provides some nice charts to show progress.
Authority Labs in another paid tool that provides fully automated reports, allowing you to track local results, international keywords, and a large volume of keywords.
Advanced Web Ranking and Rank Checker are additional services that can be useful for tracking web rankings.
There are a lot of good ways to track search engine rankings. If you have additional SEO services or tools that you like to use for tracking rankings, feel free to leave a comment.