Well the best method quality wise is using The Foundrys "Kronos" ( http://www.thefoundry.co.uk/products/kronos/ ). Its a similar tool like twixtor but delivers the better results (in my opinion).
If you dont want to use that/spend money then use twixtor OR the built-in time remapping functions, you get the best results by letting either one of them do its job on its own. Otherwise its like using sharpening on a layer, then render that out/precompose it and then use another third-party sharpening effect ontop of the old one. The result will be messy and anything but nice looking. All you get is a weird look and not a sharper image.