I've got three fairly substantial and mostly painted 40K armies, and I've learned a few simple lessons as I've painted them. I like to think they all look pretty good, though the Executioners are certainly the best. Regardless of your army or paint style, there are a few things you can do to make your army striking on the tabletop.
1) Maintain a consistent style
Every model in your army should be painted in the same style, and you should keep a consistent style even on a single model. For example, my Blood Angels are painted in a bright style with strong highlights. This gives them a high contrast, almost cartoony style.
On the other hand, my Executioners are much darker, using soft drybrushed highlights. All of the color choices are muted to provide a moody look.
The styles are obviously different and having them both in one army would be visually jarring, but that's not my whole point. If I painted any part of the Executioners in the same style I use for the Blood Angels, it would stand out like a sore thumb and distract away from the rest of the model. If I used the Executioners style on part of a Blood Angel, it would look unfinished and disappear behind the brighter parts of the model. Maintaining a consistent style, no matter what it is, over a single model makes it all come together visually.
2) Paint your bases
This one took me years to admit. I based my Blood Angels and Eldar with unpainted sand. They were supposed to be on desert bases, so what looks more like sand than sand?
As you can see here, painting the bases makes a huge difference in presentation. At this scale, the sand just doesn't have enough texture to produce its own contrast. A highlight and wash goes a long way to making it look complete. Additionally, a black edge to the base makes the model look much more like a completed showpiece. It also helps cut down on the obvious contrast if your bases don't match the table you're playing on.
3) Never be afraid to refurbish your models
If you've been reading the blog recently, you know I've been refurbishing my old Eldar. It's an unavoidable fact that your painting style will evolve as you work your way through an army. Later units will look better than early units. Sometimes, it's necessary to strip older models and start from scratch, but they can often be improved just by applying your newly learned techniques. If you didn't highlight or shade your early models, you can go back and do that over your basecoat. If you were slopply with your transitions between different areas, you can clean them up. I find that older models are usually not beyond hope. They mostly seem like they were stopped before they were finished. Going back and spending some more time on them probably won't bring them all the way up to your newer models, but it will go a long way to making your army look consistent.
4) Pay attention to the small details
If you have followed the previous guidelines, you'll have an army that looks good on the tabletop and will attract people's attention from that distance. When they decide to look more closely, you need to have some tiny details to really draw them in.
In my armies, these are mostly some sort of freehand. From the aspect sigils on my Eldar to the squad markings on my Marines, there are little details that are only obvious on closer examination. This is just because I like to paint small symbols. However, freehand isn't the only way to go. Shiny, realistic gore would fill this role on Khorne Berzerkers, while dripping saliva looks great on Tyranids. Sculpted details or conversion perform the same function if you're better at that.
These details won't be easily visible at arm's length, but they'll hold people's attention once your overall paint scheme grabs it.
There are my best tips for producing a great army, no matter what type or game. Do you have any tips of your own?