I tend to brew Belgian styles more than anything else, and one of the things I've learned is that it's extremely difficult to replicate the Belgian greats. I've tried with Westvleteren 12, Orval, Rochefort 8, Westmalle dubbel, La Chouffe, and most recently Pannepot, with varying degrees of success.
That being said, I don't think there are any real secrets to brewing a great Belgian style beer, and I don't necessarily think they are any more difficult than most styles. I would say however that in my experience the lighter beers (saison, tripel etc) are easier to get right than the darker ones (dubbel, quad etc).
Some tips off the top of my head (although
@chthon has covered it well):
1. Use a liquid yeast. I know many will disagree with me on this, but Belgian styles really need a quality yeast, and dry yeasts just aren't up to the job.
2. As mentioned, sugar is your friend. Avoid buying the hard candi sugar rocks. If the recipe calls for light candi sugar/syrup just use table sugar. If it asks for dark candi sugar then it's very simple to make your own and considerably cheaper than buying it.
3. Keep the recipe simple. Most Belgian beers have a very simple grain bill, so go easy on the speciality malts.
4. Mash nice and low. Don't worry about it being too dry, this is difficult to do with most Belgian styles whereas too sweet is a problem.
5. Don't get carried away with fermentation temperature. It's true that Belgian yeasts work well at higher temperatures but you'll get better results by pitching a little low and ramping the temperature up during fermentation.
6. If you want to try spices then coriander seed is a good one to start with. About 10g in a 20L batch will give a subtle background note which blends well with Belgian yeast flavours and doesn't stand out too much.
If you want an easy opener then I'd say a saison is a good place to start. This is my rye saison recipe which is simple but effective:
Rye Saison
Recipe Specs
Batch Size: 21 L
Original Gravity: 1.055
Final Gravity: 1.003
ABV: 6.8%
Colour: 6 SRM
Bitterness: 26 IBU
Grain Bill
3.0 kg Weyermann Pilsner Malt
1.5 kg Weyermann Rye Malt
350 g Granulated Sugar
Hop Bill
25g Styrian Golding (4.8% AA) @ 60 Mins
30g Styrian Golding @ 20 Mins
30g Styrian Golding @ 1 Min
Notes
Mash at 63°c for 90 mins
Ferment with Wyeast 3711 French Saison
Pitch at 20° and ramp up to 25° over 5 days
Carbonate to 3 vol/CO2