I am thinking about adding a rule that allows players to trade unused fate points at the end of a session for experience. It would work something like this:

At the end of a session, when experience rewards are being given the players may trade one or more fate points for 50 experience each. If this is done all players must trade in the same number of fate points.

This rule would be to encourage the players to try and save fate points. It would reward a party who was particularly clever in achieving their goals. The second part is so that players with fewer fate points are not penalised by having less opportunities for experience. What do think of this rule?

Logically, wouldn't the reverse be true? if you're rewarding unused fate points, and both Player A(Who has five fate points) and Player B(Who only has three) both have to use a fate point to avoid falling off a cliff, Player A is going to get double the bonus experience at the end of the session simply because his origin path and character creation afforded him more.

That's what the second part was for, so that players with fewer fate points aren't at a disadvantage. Technically the whole group may lose some experience if one character has used all their fate points, but that doesn't really matter as the group is still equal and encounters are scaled to the group. Plus it will add some motivation for teamwork to keep that from happening.

Players with less fate points shouldn't be at an XP disadvantage right now. You don't aware XP for rolling well, you award if for role playing, attempting cool things, and to the group for achieving objectives. And currently, as the GM you have control of balancing XP. Your system makes it so that there is now a mechanic ensuring that the less fate points one has the less XP they can make. Some one with four fate points can get 200xp per game, but someone with 2 can only get 100.

This is a team game, I would expect fate points are usually used to help the whole crew, yes? So it is hurting everyones chances.

Don't try and discourage the use of fate points. They encourage the players to take more risks, make heroic rescues, and keep one bad roll from ruining the game. Don't rob your game of it.

I usually add fate points to games even if they don't have them because they have such a beneficial effect on the players playstyles.

Again, don't discourage the use of fate points, and don't provide mechanics to force them to save them, the mechanic is already there (the unknown future). This is a huge rule change that will really throw off the balance and feel of the games.

Ah, I see the part we both missed, you said all players had to trade in the same amount of fate points.
Well than all you are doing is discouraging the use of fate points, which will have a negative effect on your game. Your game will be more fun if your players use all of there fate points every session, at least if they are using them correctly.

Basically you are robbing the story (characters doing cool things) for the gameplay mechanic (xp gain). Gameplay mechanics should ALWAYS be supporting the story, never vice versa.

My goal was to encourage the players not to use fate points. I see your point though that using fate points can allow them to perform some impressive things that may otherwise be missed. My party has so far used FP mostly for healing which isn't all that interesting. To be fair though, in most of the cases they needed more healing because they had failed another test which lead to their predicament. Perhaps they just need to think about the consequences of failing a pilot (spacecraft) by 5 degrees and re-roll that.

Ah, I see where you are coming from.

My suggestion for your situation? I don't think they are paranoid enough. My players usually try to save their FP, but it is because they are convinced they will need them.

Last night for instance they were navigating an icy ship graveyard, walking along a ledge over a thousand foot crevasse. A small quake and two of the players fail,one in front of the senchal, one behind. The senchal grabs the one in front, the one behind grabs the senchals leg, pulling all three over. The Rogue Trader misses his grab so they begin to fall. At this point the Arch-Militant uses a fate point to leap off the ledge, grab the senchal, and embed his sword into the ice to anchor them.

Another time they had fallen behind a Rival en route to a treasure. So they came up with a plan to slingshot between a binary star between flares. The planet on the other side was too close and they wouldn't have time to slow down, so they sere going to reverse sling-shot the planet and skim the upper atmosphere to use the friction and gravity as a break, hopefully slowing them down enough to off a landing ship without it getting destroyed by the friction, which would then require some challenging piloting to bring down on top of the target zone in one piece. Fate points galore were used that day.

Basically, provide them with opportunities to do cool stuff, time when they will NEED fate points to succeed. Don't plan out solutions for them; in both circumstances I had no idea how they were going to make it, but trusted that they would think of something. If they know unexpected and dangerous circumstances are likely to come up they will try to save those fate points for when they need to do something amazing. An added bonus is that they then get used to thinking outside the box, and are more likely to come up with cool things to do that will require fate points.

It sounds like your players are expecting just expecting continuous standard style combat encounters, which makes investing in wounds and healing an obvious choice. They know that wounds are going to be the most important factor in their success. If they don't know what is going to happen and what sort of obsacles they will be up against (I have played several sessions with no combat), than they will be more likely save their fate points for those situations.

fate points are what makes the pc's heroic and more than just another human in the imperium i've always found that with my campaigns fate points make for a more "cinematic" gaming experiance..... no one rembers that time they all died because they didnt use fate points but that time they almosts died but just avoided it through the use of fate points... that gets discussed for a few weeks