Oh, and a very useful trick, though I hope I'm not going too fast for you on this one:
1) Create a group for friends, your business, something like that, just make sure to set land permissions within the group pretty tight.
2) Donate your tier to the group
3) Make sure you wear the tag for that group when you buy land
4) Buy your 512 sqm plot, but Buy for Group, not for yourself. The reason comes next

5) Quickly look around the sim, and see if there are any cheap microparcels laying around, especially parcels near your newly aquired land
6) If they're relatively cheap (less than 75L$ for 16sqm be considered cheap), buy 3 of these 16 sqm micoparcels, or 48 sqm worth of them, and buy these for your group as well.
Groups get a 10% land bonus allowance on Mainland. You can't get 51sqm, but 48 sqm is the closest available. It's not much, but still, it's 9-11 extra prims. In addition, you're sure that there won't be appearing an ugly advert on that particular parcel anymore.
If you get along well with your neighbours (just make sure they're really residents), you might also be able to arrange a landswap for these microparcels, so they may attach to your main plot, or to combine your 3 microplots and let your neighbour combine his/her microplots as well.
Some sims even have arrangements where each resident sacrifices 1 or 2 prims from their 3 microplots to plant a tree or do something else nice to improve the neighbourhood.