Best way I can think of (I haven't tried this personally, but saw the guy do it in a bodyshop where I helped him work on my brother's car), is sand the rust away as best as you can, apply rust inhibitor, and then fill the chip with paint. You have to be very careful not to damage any surrounding paintwork when sanding / applying rust treatment, but if you use very fine (2500 wet and dry paper?) you should be ok.
Filling the chip, again, very carefully, building up thin layers - do not be tempted to rush it or apply 'blobs' of paint and allow the paint to dry between coats. When the final coat of paint is slightly proud of the surrounding paint, flatten the area slightly with 2500 wet and dry paper (used wet with loads of water and soap), and then polish out, preferably with a machine polisher (but that too can damage your paint if you aren't experienced in using one of them as heat builds up and burns the paint).
I'd personally be too scared to try it, but this same guy sanded out a long scratch on my boot lid which I thought would need to be painted....