For smoother fillets, add a scoop or two of Cell-o-fill (or silica) to the epoxy then thicken with wood flour to the desired thickness.  It makes the mixture more like creamy peanut butter and easier to spread.  Once the fillet is in place, lightly brush with a little unthickened epoxy for a very smooth finish.  

   riffing on what mark said above, cell-o-fill in small amounts is great for this, after a few batches I discovered that just a little bit less than what the manual recommends (thinner) was the best.

I also recommend mixing smaller batches and working slowly.

Lastly, once the fill material was solid but still tacky I put a plain coat of epoxy like mark said above.

Smoothing with a gloved finger wet with denatured alcohol after the fillet has started to set up, but is still somewhat pliable, will help a lot, as well.

.....Michael