Science illustration can pose a challenge with its conflicting demands. On the one hand, illustration is about creating something visually memorable, which is easiest achieved by making it look different to what everyone else has done before. On the other hand, science is pretty specific about what is and is not accepted as true. Unless you've been granted some artistic license on the subject, you can't just change the science of your subject to make your illustration more interesting.
The first way you can make your illustration stand out from the crowd is to adopt a particular style. Typically in science illustration, a few styles make up the majority of science illustrations. These are black and white ink figures which reproduce well in journals, airbrush illustrations that are used in anatomy textbooks, and 3D renders, which are usually stills from animations. These styles all strive for accuracy. If you can take liberties with how accurate your depiction of a subject needs to be, avoid these styles and try something new that's less precise.
If you don't have the luxury of being creative with the truth, you'll probably have to adopt one of the aforementioned styles. To make your illustration stand out, you need to see what the competition is doing. Type your subject into google images and do some picture research.
In this example we're creating an illustration of the gastrulation stage of Drosophila development. The majority of the results on google are schematic cross-sections that all look quite similar. There are also some line-art illustrations of the drosophila embryo exterior to emphasis the furrows developed from in-foldings, and some photographs of the embryos from papers. These images are all useful for reference, however we should avoid making our illustration look like them. Save them for later and refer to them when you're drawing the make sure you don't do something stupid like draw the DNA helix spiralling the wrong way (Check Google – it's a common mistake).
An easy way to make your illustration stand out from the others is to translate the illustrations with the most important information into a different number of dimensions. If everyone else draws your subject in two dimensions, add a third. If everyone else requires three dimensions to illustrate a subject, see if you can do it in two (the latter is much harder).
Another approach, particularly useful when illustrating signalling pathways, is to replace all gene names in a pathway with the closest approximation to their protein structures. Ideally there should be no text in an illustration (otherwise it's an infographic).
In this case, the transverse cross-section of the Drosophila embryo gives us the best view of the gastrulation process. This doesn't always make it clear which orientation the embryo is in relative to the section though (I had to double-check using google). Including some detail of the 3D embryo would help the viewer work out which plane the section had been taken in.
So now we have a concept for our illustration, a transverse section but with a 3D angle. The easiest way to illustrate this is to look like we've sliced our embryo into two (a bit like a sushi roll).
Now we want to arrange these two sushi slices so that the key information of the graphic, is as visible as possible (the cross-section). Ideally we should put it in the middle of the image (for good composition). We could have the two pieces in line but spaced apart slightly, but this would result in some of the detail being obscured (in the sushi image above you can't see a complete cross-section of any of the sushi pieces). Instead a better view would be the two halves side-by-side, but still at an angle so we know it's 3D. This also centres the most important information in the illustration.
So here's our illustration. We've also added a little bit of detail on the two halves to make it obvious that it's a drosophila embryo, and at what stage and orientation its in. We don't want to add too much extra detail though, otherwise we risk distracting the viewers from the important information in the illustration. The same principle can be applied to the colour scheme. Use colour sparingly, mainly to highlight the important features of the diagram and to differentiate the foreground from the background.