Modified Huffman coding that tries to behave as an adaptive one (this is NOT a so-called "Adaptive Huffman coding", but it would be it if this phrase hadn't already been reserved). This is something that does not exist (at least I did not find something like this except for the above-mentioned Adaptive Huffman coding that uses different technique, so it's not the same), so it was sort of an improvisation from my side.
Briefly: Unlike normal Huffman coding that works well for static letter frequencies only, Improved Huffman coding works well for both static and dynamic (!) letter frequencies. Worst-case (dynamic freq.) compression ratio for normal Huffman was around 1.06, when in case with Improved Huffman the worst-case compression ratio was around 1.18, which is better than normal Huffman. Considering that a best-case (static freq.) compression ratio differed for a negligible amount when comparing both codings (around 0.01 less for Improved Huffman), I can say that the efforts for Improved Huffman coding paid off.
A bit more detailed: see THIS.