What will make fruit fillings more flavorful? I often find my pie interiors a little dull.

you are hard to please, ivalleria, maybe you should stick with pumpkin and pecan.

I am usually not a fan of embellishment, and if I'm making a pie it's only with farmer's market, fresh seasonal fruit. I admit it's strange, but that's precisely the problem. I'm using the "pure" fruit and somehow feel like the filling doesn't pop. I sometimes wonder if it's precisely because those fruits are less sugary than their flavorless supermarket counterparts. My pies are often rather tart, with the prescribed amount of sugar. I like tart. But I also feel there's a missing heavenliness. I was very satisfied with the allspice and molasses in my blueberry filling. It tasted quite strong raw and then once cooked just tasted like blueberries, but more delicious. I think.

Once again too much embellishment.. Too many liqueurs, spices,unexpected ingredients for novelty's sake..won't always give you the best result.. The true clear favors of good fruit should shine through.

A jigger of liquor - amaretto in anything, peach schnapps in cherry, Grand Marnier ... You get the idea. The alcohol cooks off (mostly) but leaves just a bit of mouth feel.

This lemon-infused raw cane sugar would probably be great in a pie:
http://food52.com/blog/7221-the-best-lemonade-you-ve-ever-had

Ivalleria, my curiosity is piqued! Why wouldn't you trust them?

In case anyone is curious, I just took my hand-pies out of the oven.... used blueberry, allspice and MOLASSES (just a touch), in addition to the lemon juice the recipe called for (is it me? I feel like this never moves the needle) and I have to say they are insanely delicious. My Smitten Kitchen pie dough technique is a winner.

Fresh grated ginger and lemon zest w blueberries!

I use hefty doses of citrus (zest and juice) in my baking. It really enlivens what could otherwise be one-dimensional or cloying. Grated fresh ginger is another good one. Sometimes, if I'm feeling feisty, I'll even add hot peppers. I made a very memorable habanero-peach pie once (not memorable because it was inedibly hot or had bad--ahem--after effects, but because it was freaking delicious). Fresh herbs are another good bet, but they won't add zip like citrus or ginger. Maybe use herbs in tandem with citrus. Also make it a point to get stellar fruit. The freshest, most flavorful fruit you can get. It's an obvious suggestion, but I'm amazed at how flavorless some fruit is (supermarket produce section, I'm looking at you!).

These all sound like great suggestions. Another thing you might try is googling Food And Wine recipes for the types of pies you want to make. One of their things is to do little chef-y twists on traditional recipes -- I remember a blueberry tart recipe with candied ginger in it, for example. That might be a good source for ideas for some fun additions to your fruit pies.

Just a little lemon or lime - zest and/or juice perks up almost all fruits. And a pinch of salt works wonders, as well. Cinnamon and nutmeg are often suggested, but I find mace adds a nice note, especially to peaches and nectarines. Cardamom with pears, Allspice with plums.

A squeeze or two of lemon juice and perhaps some zest is always nice,and so is cinnamon.
Penzeys Baking spice is a wonderful mix of sweet spices that adds a sort of amazing and slightly mysterious flavor. The best advice of all is to find a source of fresh local, fruit in season. Try to avoid refrigerated super market fruit. You will notice an immediate difference .

Cardamom is also great with fruit (black or green).
So is coriander seed; untoasted and ground, it has a very citric flavor.
Also, look around for aniseed. It gives a very faint licorice/anise flavor that pairs very well with most fruits. On that flavor note, fresh tarragon is another substitute for the more common basil and mint.

I am a big fan of lemon and lime juice and zests in fruit pies. Also, fresh ginger adds a little zing.

(Typed this before I saw your hand pie response!)

If you want to try something open-faced, check out the galette article I wrote in the "No Recipe" column; I like galettes because they get direct oven heat for caramelization and the fruit doesn't stay as steamy. In terms of flavor additions generally, I like ginger and/or rosemary for blueberry, kirsch and/or noyaux (stone kernels) for sour cherry, and with strawberry I'm a sucker for balsamic. Also habanero for strawberries but not sure how that would bake since I've only done it fresh. Curious what you try out!

lemon and lemon zest do wonders for blueberry pie. I recommend cooking half the blueberries over the stove with sugar lemon zest and a touch of lemon juice and then folding in the rest of the berries when you are ready to put the pie in the oven, it also allows for more blueberries to fill the pie. I'd also recommend adding orange juice and zest to strawberry rhubarb and allowing to macerate with the sugar. Peach tastes great with an almond or pecan crust or topping and it gives a great texture. Peaches also taste great with apple pie spices- cinnamon, nutmeg and also a hint of cloves lemon zest and almond extract. Anyway sorry for the longwinded, but I think maybe you're missing a punch of citrus that will bring out the fruity flavors in the berries and cherry and then a little zip of spice or nuttiness in peach.

I was thinking mint. But of course I don't have any. I have sage and thyme. I need to invest in some almond extract. I have vanilla and lemon extract. I guess I'll add some allspice to my blueberry mix. I'm making handpies for tomorrow and have a filling prepped that isn't blowing me away.

I agree with hardlikearmour. I put lemon or lime zest and sometimes mint in blueberry pie. I also like almond with cherries, and have also used orange and lime zest in cherry pie. With peaches, nutmeg is traditional, but I really like a tiny bit of allspice. Have you tried basil with strawberries? Here's a tart recipe that I have my eye on: http://www.pbs.org/food/kitchen-vignettes/strawberry-basil-tart/

Almond extract goes perfectly with cherries. I like allspice with blueberries. You could also experiment with different herbs. I did a blueberry-apricot combo with lavender and verbena that was really nice. I pulverized the herbs with the sugar in the food processor.