Index getting "old" fast

Friday I was working on some queries that were taking an unreasonably long time to run on our 7.3.4.5 instance.

The queries in question are actually on views, which in turn a simple selects, on a table with 3 collum, a date stamp, an identifier, and a reading. I observed that the selection would benefit from a index on the identifier collum, so I create a simple non-unique index on that collum. Indeed the selects drop from ~20 seconds, to ~3 seconds.

However, this morning I observed that these queries were back to ~20 seconds. Dropping and recreating the index again speeded them up.

Data is being inserted into this table on a regular basis, as new readings ae taken.

How can I create an index that doesn't "age" so fast?

-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
						-- Benjamin Franklin