When you are investing in specific themes or trends you need to be much more active in monitoring that trend as opposed to holding quality generic domains. You need to follow both end user sales if any, and the wholesale domainer to domainer market.
Over the years there have been many of these types of domains that have spiked up and then came back down. You need to be selling into the frenzy and holding only your top few names if any when they are coming down.
The LLLL.com had people buying in bulk on Namepros as the buyout happened, names that made little sense like qwzx.com and many more like it sold in bulk packages by those who had a lot of LLLL.com and sold into the LLLL.com buyout. Bulk deals were done and the seller made a decent ROI for a reg fee domain with no obvious use. After a little while these poor LLLL.com came down to earth and were not getting $30 to $35 but more closer to reg fee.
There was a nice push for LLL.us a few years back as there were a few buyers trying to corner that market. The price of LLL.us went up nicely. After awhile the pricing came back down and now you can find some days where they are offered for $7 for some of the lesser quality LLL and even good ones go for less than when the buyout happened.
I remember that I for a little while invested in L-L-L.com, at the time LLL.com were at their peak and there was a thinking that those who could not afford an LLL.com might go for the hyphenated version. There was a lot of research done and a thread started on Namepros where members shared regs and L-L-L.com that were developed. These names were being back ordered on Namejet and went for $99 or more. I remember I paid mid $300 for a same letter L-L-L.com, that buyer was smart he sold into the frenzy, I sold it last year for about half of what I paid. I did sell some hand regs earlier for $100 to $200 so the overall experience was slightly profitable, I also did not reg too many no more than 20. Now many of these names are available as the fad has passed. There is the ocassional four figure sale of an L-L-L.com but these do not happen every month let alone weekly.
There are other trend domains like 3d and cloud and tablet, and unless you own category defining domains you need to be ready to move fast. The first rule would never be to put more than a few dollars you can easily afford into these types of names as a casual domainer, unless you work and or know the niche better than the average domainer and have the speculative budget.