Really really take your time adding the sugar - the slower, the better, it should take about 8-10 mins.
It is highly unlikely that the meringue can be over whisked - I thought it could.
Don't skip the sifting of the icing sugar - any lumps will knock the air out of the meringue. Again, I learnt the hard way that this bit is important.
Other flavours that can be used are vanilla extract, peppermint extract, orange extract, cocoa.
I use disposable icing bags as they are easier to handle than baking paper bags.
I use the star nozzle and leave quite a peak when I pipe them.