When it comes to encouraging a customer or client to spend more, the incentive for them is normally that they will get better value.
For instance in a supermarket, when buying a milk one particular carton of milk, if you notice that a bigger carton of milk gives you more milk per penny then you may well be tempted to spend more and get that bigger carton.
Reflecting that back on massage if you charge $50 for 1 hour but you would really like your client to upgrade to 90 minutes you can help ease their decision by having a price that shows 90 minutes has more value. So instead of $75 for 90 minutes you might make it $70 or maybe even $69.
The other way of encouraging a client to opt for the longer session is to include some special extra, which might be
1) A free mini Indian Head Massage or...
2) A sample bottle of lotion or body cream (you can often get sample's for free for your clients by contacting the companies that make them) or...
3) A free relaxation recording from a hypnotherapist who you have networked with (the hypnotherapist gets to put their promotional details on the label of the hypnosis cd and you get a value product to give away) or...
4) Anything else you can think of to make your longer session unique and attractive
I hope that gives you some more ideas
Best wishes