you should have done a poll but i guess youre after a little more information
domains arent normally a regular form of income so you'll have to adopt a number of tehniques if you want to stay lucrative.
Some domains you'll flip straight away - its a good model if you catch, as the most interest comes as they are about to drop as likely owners line up to try their luck.
Some you'll choose to keep, with percieved potential of the domain is thought to be more than the the trade prices offered for a quick flip.
These days parking isnt a fesiable option for income - half your portfolio may make money but it'llbe carrying the other half. Look to get some unique content on there and develop a simple template. But parking is EASY - put the domain up and earn instantly - if only a few pennies.
Develop is probably the most lucrative, all big sales are normally a combination of GOOD domain name and established traffic but the process can be long and hard work, ,most domainers are either one or the other. (developer / investor)
Bsaically it comes down to funds. If youre sitting on a hedge fund style bank balance then buy buy buy the sales will come. if youre normal like the rest of us then look for the oppertunity, take the money when the roof over your head is at risk.
Competition is widespread so play to your strengths.