Tip 1:
You can make some good use of the holga's flash to make some creative effects! The color flash on the camera will only reflect on subject that are near. So a shot near a window will have an effect as shown :)
Alternatively, if you shoot with the red flash on a human subject near you, this is what you will get! The creativity is yours :)
Tip 2:
Make full use of the Bulb function of the 'N' type Holga to take some creative night and low light shots. Switch to 'B' and the aperture will open as long as you hold the shutter button. How long you hold depends on how much ambient light is available and what effects you like to achieve.
Here is an example of my long exposure on the street lamps taken in a car, from the passenger seat. Make sure you do not distract or obstruct the driver!
There are many other dramatic and creative effect that can be achieved using the bulb function.
Tip 3:
Shoot using the 35mm film instead of the medium format. This method require some modifications to your Holga but it is really not too difficult. The modification procedures can be easily found from searching the internet.
These are some of my shots with the 35mm film on my Holga:
More interestingly, you can buy a film scanner, and you can produce pictures with the sprockets! Well, I would say this is more challenging and a relatively good scanner should be used :)