Things to look out for when buying SEO services?

I'm looking for some solid and reliable SEO services, at the least, that won't get my site penalized by Google. After reading Google's opinions on unnatural links, I'm starting to get paranoid.

I mean ain't most of the SEO services out there offering and building links in these ways? Have you had any experiences with SEO firms / providers that get your site actually penalized in the end for using these ways to build links?

I'm looking for long-term results, not just short-term gains, so the last thing I want is to spend money to get my site penalized after a short outburst. I checked out quite some SEO providers all they claim are:

- 1. How they can build xxx links, xxx directory submissions, xxx articles published, xxx EDU/GOV links, xxx blog comments, xxx forum profiles / posts / signatures, xxx social bookmarks, xxx guestbook entries, across a matter of 3 - 6 months.
- 2. How they can bring up rankings in a relatively short period of time, e.g. 1-3 month ---- it's great but I had my own experience when my site gets up in the rankings quickly then penalized forever.

According to Google's link scheme standard, these all fall in that kind of efforts that will be negative to SEO rankings, do they?

I used to do SEO myself and these ways actually work a few years ago. I haven't done any SEO in recent years so I'm not sure if they work now or if they actually get penalized by Google as they state.

What's your recent experience? If they don't work now or even work negatively, what should I do now in SEO?

I get offers ranging from hundreds of dollars to thousands of dollars per month, is more expensive the better? What should I look for in an SEO firm from now on?

Would really like some experienced pros here to share some insights. Thanks!