A couple options you can try for tricky "between board and socket" soldering:
1) use an iron (such as Hakko) with replaceable tips, and use a tip with a fine, angled needlepoint. This can allow you to fit the tip in tricky areas with tight surrounding clearances
2) On more of a case-by-case basis, it's possible to break machine-pin SIPs apart and/or cut the plastic away from a single pin. You then install the socket in "sections". Use an old IC to keep your new smaller SIP sections aligned as you place them in the board. Then apply Flux to the exposed pin and board pad, and attempt to flow the solder in similar fashion. At any rate, by breaking the plastic away from the pin you're giving yourself extra visibility and access which just might be enough. You can even try it with closed-frame sockets too: simply remove only what's in the way. In all cases pay attention to the surroundings and make sure to keep the socket pins aligned.