Modeling pronunciation variation is key for recognizing conversational speech. Previous efforts on pronunciation modeling by modifying dictionaries only yielded marginal improvement. Due to complex interaction between dictionaries and acoustic models, we believe a pronunciation modeling scheme is plausible only when closely coupled with the underlying acoustic model. This paper explores the use of flexible parameter tying for pronunciation modeling. In particular, two new techniques are investigated: Gaussian tying and flexible tree clustering. We report a 1.3% absolute WER improvement over the traditional modeling framework on the Switchboard task.