Fig 1. Inclinometer mounted on a paint stirring stick
One way to measure dip is using a fluid-damped inclinometer from Lee Valley Tools (C$30, Ottawa).
To keep the inclinometer from sinking into soft surfaces (eg., sand) and parallel to a slope, I mounted it on a paint stirring stick (about 30cm x 3cm x 0.5cm). The inclinometer has a magnetic mount, so I glued a flat piece of metal (cut from a can lid) to the stick as a mounting point, laid flush on the wood and then glued at its corners to ensure it rests flat on the wood. The metal plate is offset length-wise such that the center of the inclinometer is over the middle of the stick when on a 30 degree slope (close to typical angles for sand slopes), as shown below:
Fig 2. On a sand slope
On a stick 30.5cm long, I centered the metal plate at 13.4cm. Putting the center of gravity over the center of the stick when on a sand slope helps keep the stick from digging into the soft sand.
Uniform weight distribution also helps to keep the stick from tobagganing down sand slopes (especially on slopes at the angle of repose, where sand is already on the verge of sliding). To provide friction, a piece of terry cloth towel was glued to the base of the stick (using a hot glue gun, taking care not to introduce an angle). This arrangement works well; even above-angle-of-repose slopes have been measured without triggering avalanching.
The device can be tested by measuring a known-level surface, or by just measuring any slope twice, reversing the device between measurements (the two measurements should be the same).
The inclinometer can be enclosed in a plastic ziplock bag, even while mounted, if sand has magnetic material (but in practice this hasn't been a problem).
Fig 3. Inclinometer on sandstone
Below (fig 4), the inclinometer stick is in a trench, resting on two bamboo stakes that have been inserted in the trench wall along a lamina, to measure its slope. The inclination can be read using a mirror, or using a digital camera. Of course it's important that the stakes be level (or at least consistent).
Fig 4. Inclinometer in a trench