Reduce the watering from tomatoes in a dish or sauce
Simmering

Let tomatoes simmer slowly for hours.

Pros: probably the tastiest solution.

Cons: time consuming, energy (gas/power) consuming, loss of "fresh" tomato flavor, if you want that.

Adding thickener

Add flour/bread/carrots/potatoes/whatever to add consistency.

Pros: quick and easy.

Cons: may change the flavour.  May require heating to near-boil to activate the starch.  

Seeding

Remove seeds and gel/pulp with fingers or a spoon.

Pros: quick, and removes seeds, which may give a bitter taste.

Cons: removes pulp (which gives most of the 'umami' flavor) and juice.

Filtering

Use a strainer to remove the watery part.

Pros: ?

Cons: removes part of the juice.

Draining

Drain (without seeding) tomatoes before to bake them - a (very) light salting helps to draw out excess liquid.

Pros: Removing liquid before allows for less cooking time and preservation of fresh tomato flavor

Cons: waiting time, and removes part of the juice.

Roasting

Halve and place on a sheet pan, then bake until sufficiently dewatered for your purposes.  May have to leave oven door cracked open.

Pros : Easy.  Skins are easy to peel after roasting.  Adds some caramel/roasted flavors where the sugars burned.

Cons : Not quick.  Not good if you want skins on.  Loses the 'fresh tomato' flavor.

Dehydrating

Bake as you would to create a sun dried tomato (at low heat) but don't put it in olive oil.  Can reduce the time to not remove as much moisture.

Pros : Easy.

Cons : Very slow (we're talking most of the day slow).  Loss of the fresh tomato flavor.

Tomato Selection

Use a plum tomato, or other variant that has a higher flesh to gel ratio.

Pros : Extremely easy (if you can get them at your market)

Cons : Often have less flavor than beefsteak, heirloom and other varieties (but may not be noticeable if you're buying from a grocery store)