The key?

Don't add the Splenda before whipping.

Whip it until stiff peaks, then add splenda, then finish whipping.

Anything (davinci, splenda, et al) will make the cream have less "staying power".

Even better -- get yourself an ISI cream whipper and you won't have to worry about any of this.

http://www.kitchenetc.com/Products.cfm?sku=000106423