Since I speak Spanish, most conventional forces doesn't work for me, but I studided some basics and principles that have helped me so far:
Most people when told to think of a card, they do not create the card in that moment, they are going back into their minds to recolect information so that thety can make up a card. A suggestion is where you fill those holes giving their minds the information right away, so that they don't go back in to recover a memory in order to make up a card.
For example. Do a card trick, do a couple of other tricks that doesn't involve cards, and then ask them to think a card and then to tell you that card. You'll find that aprox. 45% of people will tell you the card they saw during the trick. Or something really really similar, like the same number with a diferent symbol and things like that.
The force that I use is the following, it works like 60% percent of the time (But I still got my outs in case they fail)
You start by asking them to think of a suit like this:
"Do you know the four suits right? Spades, diamonds, CLUBS ( I snap my fingers at this point and I make a little emphasis on the word) and hearts. Think one of them now "( I snap my fingers. I used the snapping before so that I could link in their minds the CLUBS with the snapping).
They will mostly not think about the suits on the ends, and because of the little emphasis, they will think in that suit most of the time.
Now the number. Because I speak spanish, I usually force the 3 of clubs this way:
"Necesito que te concenTRES (Also showing to their periferial vision the number three with my fingers.) so that you cna come up with a number (I draw the three in the air).
In english there are some words that have the numbers spelled into their letters, I don't remember one right now, but you can use the same principle. O yes, I remember one " Do me a favour and think a number FOUR your suit"
Things like that.
I also flash the three of clubs a little bit at the bottom of the deck in the beggining, so that image goes straight to their memory bank.
It has worked somewhat efficiently for me.
Further more you can check this products for Psy forces of a card in english:
http://www.deechristophermagic.com/anate.cfm
http://www.danielmadison.co.uk/Psycho.html
I Hope that helps