Hi Fred,
These one way links should not have an affect on your ranking. When linking sites try linking into your site and from sites that are highly positioned in the search engines and that have lots of links themselves, i.e. directory sites. As mentioned before, if it does not benefit the search engines at least it gets people to find your site (which is the aim of the exercise anyway.)
Helen
Sorry Helen but I couldn't disagree more. I will be the first to admit that I am not an SEO guru , I follow some blogs (such as Matt Cutts) etc to keep me as up to date as I need to be from a design and development perspective. I am sure a couple of our resident SEO experts will be along shortly with their professional take in this....
Back in 2006 (I think!) Google made some changes to their algorythm that depreciated the value of reciprocal links. This was due to an increase in directory based sites that added little to no value to the authority of the site being linked to. It is also my inderstanding that the majority of directory sites are of no real benefit these days, thats not to say all - but most.
If you think about it, it does kind of make sense. If 500 different websites are linking to you independantly via one-way links your site is being recommended as a useful resource, especially if these sites are related to your field or subject. If you were to have 500 reciprocal links not only would your site be jam packed full of links leaving little room for content, but all it shows is that you managed to contact 500 people and arrange for a link to be put into place in order to try and improve your ranking.
There is value in reciprocal linking if it provides an improved user experience or benefits the visitors to your site. If your looking for improved search engine placement the focus should be (in my opinion anyway) one-way incoming links from reputable, authoritive sites related to the field/subject area of the site concerned.
That's my take on things anyway, I am sure quite rightly others will have their own.