how to convert TIFF to ECW ?

If you get any error messages you may find that your tiff is REAL32 which isnt supported by ECW. There is a work around though using the raster interpretation coercer. Basically forcing the data through as 8-bit.

Here is a new article if you are looking for more information: How to Convert GeoTIFF to ECW. If you are just doing a simple conversion, you can convert directly from GeoTIFF to the ECW using the ER Mapper ECW Writer, but if you are having issues with REAL32, as mentioned by jgillespie, this article covers that more in depth.