I am writing a descriptive essay about what my life will be like 30 yrs from now. Which tense should I use? "I will be wearing/wear the same boyish clothes I wear now". I mean, I am speaking of a habit in the future.
Both are possible, and there is not much difference in meaning.
Saying "I wear boyish clothes" talks about your general style. "I am wearing boyish clothes" talks about the particular clothes you have on right now.
I wear boyish clothes to school every day, but right now I am wearing a long dress because I am at a party.
So putting this in the future has the same meaning. "I will wear..." to talk about what your general fashion sense will be. "I will be wearing" to talk about the clothes that you have at a particular time.
In 30 years time I'll be wearing a long black dress as I walk up to collect my 10th Oscar. But usually I'll just wear the same boyish clothes that I wear now.
The same structure can also be used to express "determination": "No matter what people think, I will wear the same boyish clothes."
The future progressive primarily suggests what is actually going to be happening at a relatively precise point in the future. When talking about somewhere that far in the future, it needn't be terribly precise, but it would certainly be appropriate for a period of a day (say, "on this day 30 years in the future"), and less likely for referring to a whole year-long period (say, "in 2049").
The future simple, when talking about that far in the future, would be used for two main scenarios, I think. One is making a prediction of a series of events, like:
I will finish my book, then I will take a holiday somewhere exotic.
The other is refer to habitual things, much as you might use the present simple:
I will wear comfortable clothes, and enjoy a steady income.
That said, it also depends on the verb in question - while you might use either "I will wear comfortable clothes" or "I will be wearing comfortable clothes", some sentences will only work with one or the other. Don't try:
I will be enjoying a steady income.
Most things will work with future simple, and some thing won't work with future progressive.
In a lot of cases, though, it will be a stylistic choice. Even having said that one primarily suggests one thing, and the other would be used for another, those aren't rules. They're factors to think about. You can talk about predicted habits in the future progressive, and things happening on a specific day in the future simple. Apart from the combinations that just don't work, it's not going to matter a huge amount.