Best way to plan a build?

I was wondering what everyone's strategies are to start a character build. Whenever I try to start one on mids it just seems chaotic and expensive and takes forever and I still end up getting nowhere.

Ask for advice in the appropriate archetype forum. You'll learn more than you really wanted to know.

Play through a few levels, paying attention to what works and what doesn't.

Figure out what you want to do FIRST (pump up the strong things or cover the weaknesses or whatever), then figure out what will give it to you.

This is what works for me:

First, I come up with a character concept. This doesn't have to be a whole backstory or anything like that. Do I want to smash stuff? Do I want a support character? I know, I want to stab people from behind! I'll make a new Stalker! That sort of thing.

Next, I look at the available powersets and see what fits the concept best. Backstabby... well, that rules out Energy Melee, Electric Melee, Martial Arts... Broadsword looks like fun. I'll go with that. (Do not be afraid to start the character and look at the power animations and such). For the secondary.... do I want to avoid getting hit, or do I want to be able to take a hit or two? I prefer not getting hit in the first place, so I'll go with Ninjitsu (helps that it has a heal!)

Next I choose powers that I want to use. I try to keep in mind that around level 22-25 I'll have a feel for how I play the character and will likely use the secondary build to remake the character more to my tastes, so I don't plan a 'set' build at first. That comes later.

Once you're ready to pick up sets you have to ask yourself a few questions:

1) Do I go for set bonuses over enhancement values? (there isn't always a trade-off, but for some sets there is)
2) What set bonuses are most important for this build?
3) How much time/inf am I willing to spend on purchasing/earning sets for this character?

I can't answer those questions for you, but I often find myself reaching for Crushing Impacts for my melee characters, Touch of Death (I think that's the name) for some others for the +melee def.

When you find a set you liked on paper that is too expensive, either remember it or jot it down as such. You'll get used to it after a while.

If you're low level and frankenslotting, this gets easy since you're not going for bonuses. You can just buy mele A/D from whatever.

Eventually you'll learn little combos that work, like in PBAoE to get EN def, first three of the first set, 2, 4, and 5 of the second gets you nearly 5% to each. And Cheeeep and good stats too.

Best answer, it takes a while to learn. When you've slotted Kinetic Combat in four powers and you think it's too expensive, switch them to Haymaker and cry that it's only half the melee bonus, but that's life.

Stick with mids. Most people don't use it and if you IO, especially at midlevel, you'll really stand out.

I get to 50, then see what I've got.

I don't use any of the build planners. I like to feel my way as I develop a new build. That said, over time I've worked out that all of my characters' builds have a few common foundation elements.

I want at least 3 powers that can be used as attacks regardless of archetype. The game typically provides this as your first 2 single target powers followed by an AOE of some sort.

I next want to pick up 2-3 archetype specific powers, ie armors or buff/debuffs.

During this period, I've got too many powers and not enough enhancement slots. So I try to mix in some power selections that don't require slotting or, at least, immediate slotting. The Fitness and Travel power pools help in this area and, of course, I do want them anyway.

With a completely unfamiliar powerset, I will try to pick up every power in it just to see how it works. I'll try to use it often to see how it fits in with my playstyle. Attack powers are pretty simple to evaluate. I'll keep an eye out for each power's effect on the enemies to see if I need to compensate for anything quirks it has that might hinder a team. I will give each power my typical slotting initially and get a feel for whether it needs tweaking for more endurance reduction or recharge. Buffing, debuffing, ground patches, rains and pet summons may have some subtle aspects that won't become apparent right away.

Then at around level 40+, when I'm down to only decisions on Epic pool powers, I'll start thinking about doing a respec and swapping out any powers I didn't like or might want to have sooner for exemplaring purposes.

That's pretty much it.

I just re-read your post. If your question was more in regards to planning for and implementing a Set IO'ed build, then it's is more complicated. It will depend greatly on how much Inf you have to use and the availability of the pieces you're looking for in the Market, which in turn may depend on how much playtime you can put in to accummulate Merits or AE Tickets.

My general Set IO build suggestion is to not try to do everything at once. Focus on one or two related powers and try to get the Set pieces you want. Note: you may have to lower your expectations and settle for "good enough" pieces for a while. If you have alts that you don't play much, use them as buyers and place bids across a range. Craft and store at your personal SG Base until you're ready to commit to a respec to slot them in. Use a respec to reslot so you can cash out on the old enhancements that you're not keeping.