Collecting by themes is a popular way to add challenge and value to your effort. While few of us ever plan to sell our collection, an intelligently assembled set of unique items will be more appealing to buyers and friends alike. There are a nearly unlimited number of ways to theme your collection, so I've pick a few common ones to get you started. The important thing is that you collect items you like and think belong together.
Single Source Collection
This approach involves collecting as many items from the same place as possible. The source may be a vendor (like the collection of Timeless Traditions bottles at right), a single style (like my set of Egyptian bottles), or type of creation (like artist made bottles from around the country), or a location and time (such as Roman bottles from 100 to 300 A.D.).
The strength of this approach is that you can develop a comprehensive perspective on your source. For example, in my Timeless Traditions collection, I have bottles of the same style and finish from different runs. The subtle changes over time chronicle the styles development over time. If collecting historic bottles, you'll find that few are undamaged. within the same time period, you can seek out different types of damage, chronicling the effects of time and human interaction.
If you are seeking challenge and lots of research, this approach may be the one for you. A collection that contains a sample of from each source and period provides a remarkable insight into the historical development of the lachrymatory. The challenge is finding the less obvious resources. This website includes links to many resources, but there are many areas we haven't yet investigated. I've heard of lachrymatory references in many cultures, including Native American and Chinese, but haven't found any substantial support. The process would involve researching each cultures history, evaluating their mourning and celebration practices, finding collections of their heritage, determining if lachrymatory existed as legend or reality, and finding items available for purchase could be a life's work. But, its good to have a hobby.
No matter what approach you take, I'm sure you'll have a great experience building your personal collection.