The illusion of space in a painting is created by perspective and tone. The horizon is always at eye level and all lines leading away from the eye eventually meet at the horizon. Similar sized objects, as they recede, look smaller and lighter in tone in relation to each other. Thus objects in the foreground appear larger than their equivalents in the background. The viewer has no problem believing this relationship because they have seen it in real life, so you only need put the suggestion into their mind to enhance the perception of space in your painting. The greater the difference in size the more distance suggested.
Use your subject as the excuse to paint a watercolour, rather than your watercolour as a medium to paint the subject. The appearance of the watercolour on the paper is way more important than likeness. Lifeness, not likeness, should be the aim of your painting. However well you paint the sun or an apple it can neither warm nor feed you - a painting is not pretending to be something, it is something: something other than what it represents. Freed from the obligation to make a likeness, the painter creates something new that did not exist before: a new lifeness.
Imagine you are allowed only 50 brushstrokes per watercolour painting. You would take care to get each one right, you would make sure that each time you laid your brush down it carried the desired colour to the chosen place and made the required shape – no dithering with meaningless marks or insipid layers of colour. You would spend more time mixing in the palette and more time looking at the subject. In a successful watercolour more time is spent off the paper than on it!
Paint the way you paint.
Do not force a style or manner. By all means learn the fine art and craft of painting by copying other artists, but do not be fooled into thinking imitation is your creativity, it is your classroom. Having learned, take the courage to paint from life the things that interest you, then you will make something you do not recognise, something new and surprising that you, and others, have not seen before. Oscar Wilde puts it perfectly: "Art begins where imitation ends".
Light against dark, dark against light.
If your paintings look flat or lifeless, it could be simply a matter of increasing the contrast between adjacent tones. When I say "light against dark/dark against light", I mean "lighter than", or "darker than" - the difference in value does not have to be great to have the desired effect. Form, space and depth are registered by the eye through variations in light and shade, so too the painter uses variations in value/tone/light and shade to imply the existence of the third dimension on the flat 2 dimensional surface of the painting.
If you have a digital camera you can readily check whether the tonal values are working in your painting: take a photo in black and white, if it makes sense in terms of form, space and depth, they work!
In Watercolour, Less is more, both in the painting and in the materials.
You do not need many materials to paint good watercolours. All you need is actually one fairly large or medium sized sable brush with a good tip, three to six tubes of paint, a palette to mix on, watercolour paper (preferably made of 100% cotton), water and towel. Keep it simple, do not encumber yourself with too much choice.