Having done a few rounds with the planners (not Sussex but it's the same everywhere), the only advice I can give you is:
1. Go through their pre-application process. It is normally free.
2. Read all their policies and make sure you highlight the relevant favourable points in your application.
3. Although policies should be consistently applied across the council area they are not. Each officer will have a different opinion on 'policy'.
4. Do your research on approval given for the "how on earth did they get planning for that" categories and try and get that officer to do your pre-application.
5. If refused always appeal - it's free. Refer to similar type applications that were refused then granted on appeal.
The whole planning process is a bit of a lottery and really shouldn't be. I always liken it to refereeing - inconsistent
If all else fails stick £2,000 in and envelope and bobs your uncle!