It's an interesting question.  I type personal letters to friends abroad often.

'Friendly' should include, IMO, generous spacing and clarity, and a tidy but informal approach.  I think the appearance of the whole thing is improved by 'topping & tailing': writing the salutation and the closing by hand with a pen.  Ditto any post script.  Personal letters may be kept for many years, so I am careful about noting the date and the place from which the letter is sent.  Apart from these things, maintaining the same paragraph indent throughout and making sure that the space between paragraphs is consistent at twice the line spacing, I think that's it.

More generally, I think it is never good merely to ramble.  Try to imagine some over-arching form to the letter.  If it is possible to form some sequential narrative which moves towards a resolution towards the end of the letter, so much the better.

Formality or 'standard forms' should not prevent you from speaking your mind - that is why you are writing, after all, and not merely to fulfil an obligation or use up paper - friendliness requires some openness.

No real idea if this is what you were looking for, but IME there is no 'form', layout or practice which is recognized as 'friendly'.  Others here may have different answers, of course.

Hi Dan

​As Beak says, there is no real set format for an informal letter. That being said though, we were taught in junior school (1968 - 1972) the difference between an informal handwritten letter an a formal handwritten letter. Being a creature of habit, I've carried this teaching over to my typing as follows;

​Informal letter settings. Left margin = 1", right margin = ¾" (as you usually return just after the bell and almost never hit the right margin). Header = 1¼", footer 1". Set a tab 20 spaces from the right margin for your (return) address and start the top line of your address at the tab. Start second line at tab + 1 character space, 3rd line at tab + 2 and so on. The date goes under the return address starting at tab, a line space between address and date is optional. 

​Space 2 lines between date and salutation, salutation starts at left margin. Space 2 lines under salutation to start first paragraph and indent first line of every paragraph 5 character spaces. Sign off (yours sincerely) is also indented 5 spaces with a comma after. If you desire, you can also type your name about 6 line spaces below the sign off and indented the same so there is adequate room to sign the letter. My hand writing is not pretty, so I type my name as well as signing. Hope this gives you something to work with,

Sky

I don't do the return address on informal letters to people I know, as it is already on the envelope, takes up space, and I consider it unnecessary. I will put the city and state above the date if I am writing from somewhere other than my home. If you are writing from your home, to someone you know, the date is all that is needed. It is implied that you are writing from home unless another location is given. I justify the date to the right margin, just because I like the way it looks and serves as a good visual guide to where the margin ends. I do this by going to the end, and then reciting the characters as I backspace.

How I space the lines varies depending on the amount of text. Ideally, I would do about an inch from the top on the date, triple space to the salutation, double space to the body, double between paragraphs, and triple to the closing (sign off), and use a hand signature. I put my closing slightly to the right of center. If I want to squeeze more onto one page, I will sometimes cut out some of the spacing, first after the date, and if I need more, I will just single space between paragraphs. I indent either way. I always use one inch margins.

The recommended styles and formatting of both formal and informal letters vary between texts and evolved over time. Old typing and etiquette books can give you good examples of the rules and recommendations; as can looking at real examples of vintage letters. From there you can tailor your own style within the generally accepted parameters.

Thanks for those links. I've never seen the informal format like that before. (One could say that having a "formal" format for an "informal" letter is a contradiction in terms!)

I am sure your wife has some interesting observations from a career's worth of preparing correspondence.