Gil, great guide, totally sound advice.
Lemme add one thing. I have recently started playing on Virtue and Protector from Infinity, and what I have found really helpful in the infl/enh situation is Respecing all the way up. It can really take the edge off having to slog for cash on your way up to around 30ish when(out of sg mode) you can start to pull in some decent dollars.
I'm playing heroes, so there is one more respec trial than villains, but also I have a freebee left over from i8 and my Vet. Granted, not everyone has the Vet 'specs, but still, running trial and doing respec's at new enh lvls can help a lot. 
Actual Location: Inside the system itself.
"When the most exciting thing about Mace is AceMace's avatar, then it's time to get realistic."
Shield Guide
The only thing anyone in this game is "supposed to be doing" is having fun. Everything else is negotiable. -Jet Boy
Agreed, sound advice.
I also will add one thing that's probably been said elsewhere: strategic respec.
If you know you're going to respec, or if you have a spare respec hanging around, use it when you're ready to upgrade your enhancements (say, level 22, 27, etc., when you can just use the next enhancement level). Don't slot any enhancements during the respec--just sell them all. It's helpful if you have a tray full of stuff you wanted to sell as well. You'll come out of the respec and subsequent upgrade with a bunch more inf left over.
This is especially useful if you have an Issue freespec and the next Issue is about to hit--since you don't accumulate freespecs (except the VR ones).
Champion-server main alts:
Fraulein Mental -- 53 Mind/Kin CTRL -- 908 badges
Sunset Smash -- 50 WP/SS
Per the details of Dr Impossible's reply: Do your respecs just before upgrading your enhancements.
DO NOT SLOT any enhancements, save for those of the level you're upgrading to (Don't even put them in your enhancement tray!). Use the Inf from those to buy a fresh set. Earn the Respec whenever you feel like it, but save that sucker until you need the Inf, if you can get away with it.
For better returns, do this with a full tray of found SOs.
EDIT: Alas, I am beaten to the punch!
This guide is for those that are either not part of an sg/vg or their sg/vg does not forced to fight in sg/vg mode all of the time. If you have any guides or extra information of how you can handle your influence/infamy needs fighting in sg/vg mode all of the time feel free to add it to this guide.
1.) Only buy what you absolutely need. Meaning if you toon must have certain enhancements for survival then buy them.
2.) Do not buy to keep all of your enhancements green.
3.) Change your enhancements every five levels. You can either chose to do so at levels like 13,18, 23, ...etc, or you can chose levels 12,17, 22, ...etc. Right now I have two level 18 blasters and I have all DO's in accuracy and TO's in the rest. I will keep on going to see if I can change the DO's to SO's and the TO's to DO's by 22. I may have to wait for a full SO set at either level 27 or 32. I know that I will have a full set of SO's by level 32.
4.) Sell enhancements at the proper stores. Sell as soon as you get a full tray unless you are on a team or on a timed mission. Avoid deleting, the reason is because almost every time I have deleted a TO to open room for a DO or SO I have gotten a TO of lesser value.
5.) Exemplar out of sg/vg mode if possible.
6.) Do lower level tasks forces out of sg/vg mode if possible.
7.) Play one level in sg/vg mode and one level out, starting at level 25 where you start losing influence if possible.
8.) Use enhancements from drops.
9.) Leave the mission as soon as you have completed it. Do not stick around to clean up the rest of enemies left. Usually the amount of enemies left are worth less then the mission bonus. You are trying to get as many mission bonuses per level as possible. Remember if you do run out of content concerning story arcs, then do safeguard missions to cover the gap.
10.) Do not frequently change your difficulty settings.
11.) Do not frequently change your costumes.
12.) This is an option of last resort. Intentially go into perma debt untill either you cannot stand it anymore or you have the influence that you need. This along with doing alot of exemplaring is an option I have not had to use.
By using all of the rest I have my main that is a level 38 scrapper that has had a boost of 800,000 influence about the first few levels he started in the sg he is in. But since then has not needed any more boost to infuence. His current influence is about four million. I was able to easily afford an full tray of level 40 SO's at level 37 and had about 1,600,000 influence left over.
If I have missed anything or you know how to handle gaining influence while playing in sg/vg mode all of the time please add that information here or give a link that would guide us to that information.
Ebony Fists: Level 50 DM/Regen Scrapper, Gloom Piston Robotics/Dark mastermind level 34, QueenFireMare: Level 34 Fire blaster (pure fire),