Make sure you use your monogrammed paper. If you don't have this, use paper with your emblem, your icon, logo etc. Just something that reminds people of the unique "you". Don't forget to lighty scent it with your signature perfurme before sealing. Seal it with a wax stamp, a sticker or a kiss!
For stationery, I suggest Smythson http://www.smythson.com/SmythsonSite/pages/home/default.asp
and of course Tiffany & co. http://www.tiffany.com/Shopping/CategoryBrowse.aspx?search=1&search_params=s+1-p+1-c+-r+-x+-n+12-ri+-ni+1-t+stationery
Now, remember that writing on such beautiful stationery you will need to use your signature colour pen. Never use a boring old navy blue ball point pen, it's just not your style and it almost hurts to see such a lovely gesture tarnished by one of them. If you don't have a lovely signature-coloured pen engraved with your name, look here to find one http://www.thepencompany.co.uk/