Yeah, the SEP is a great tool for those of us who are self-employed.
One important thing to be mindful of for one's longterm retirement plan is to start as early as possible.
Besides being diversified and investing in quality investments, one should also generally view the younger years as the time to be most aggressive with your investments - when you're decades away from retirement, hopefully you can make double digit returns most years, or at least 7-8% a year on average. The power of compounding works wonders over time.
Too many people put off even thinking about retirement until they get well into their 40's or even later. Too many people live far too high, spend way too much, borrow too much, and don't have priorities in order. If it comes down to your last $1k a month of your monthly budget being divided up between your automobile of choice and your retirement investing, don't buy a car that costs $1k a month. Buy a car that will cost you $500 a month or less, and put at least $500 a month into retirement.
An old guy I know retired in his 50's after 35 years of working as an hourly blue collar worker, making an average of $80k a year. He lived like he made $50k a year, and put the rest into retirement. He was worth over $3 million when he retired. Another old guy I know started an industrial cleaning business right out of high school, worked hard, paid himself a modest salary, and sold his business 30 years later for an after tax profit of $30 million before he was 50 years old.
As you get closer to retirement, it's probably a good idea to be somewhat more conservative, particularly if you don't have a huge amount of money by the time you reach that last decade or so prior to retirement.
Hopefully by the time you retire, you have all debt paid off, all kids out of the house (or at least pulling their weight if still at home), and all of their educations taken care of (529 plans, etc). Some people downsize when they retire - they no longer need a 5 bedroom million dollar home after the 3 kids are gone, so they downsize into a smaller home and invest the rest of the money from the sale of the big house.
If you retire with $10 million or more and no debt, you can probably play around with some of your money and keep a little in riskier investments. If you retire with $1-2 mil or less, you will most likely need to make every dollar count, and will probably want to be pretty much entirely in things like investment grade bonds and large cap high-dividend blue chip stocks in retirement.
Some very conservative retired folks may keep the vast majority of their $ in bonds, while some more aggressive retired folks with plenty of money to play with may continue to keep most of their money in various stocks. Investing is a highly individual thing, and more money gives one more flexibility - particularly if they are more or less debt-free and don't have huge living expenses.
A general rule of thumb is that you should have at least $100k saved up for every $5k of annual income you'll need. If you're taking out more than 5% a year, you're probably going to be eating into your principal or investing your retirement nestegg too aggressively. If you're not earning at least 4-5% a year in interest, dividends, etc, you're going to have a hard time outpacing inflation.
A good thing about contributing a fixed amount to your retirement accounts each payday is that you are dollar cost averaging - buying more shares of your investments when the investments are down, fewer shares when they are up. This also helps when we go through periods like Q4 '07 through Q1 '09.