The concluding passuk in Parshas Acharei Mos warns us to be careful not to violate any of the instructions of the Torah. The Gemara (Yevamos 21b) understood this passuk with an additional level of interpretation: we should add a "protective fence" around the Torah laws in order see to it that we do not violate any Torah prohibitions. The opening Mishna in Pirkei Avos quotes that this was one of the three mottos of the Anshei Kenesses Hagedolah.
It is well known that most of the rabbinical gezeiros were introduced during the period of the Second Temple. The Gemara (Rosh Hashana 29b) originally raised the possibility that not blowing the shofar when Rosh Hashana falls out on Shabbos might be a Biblical law. Rosh Hashana is sometimes described in the Chumash as a day of blowing shofar, and sometimes is described as "zichron teruah - a day on which we speak about the shofar." The Gemara originally thought that when Rosh Hashana falls out on Shabbos we only speak about shofar without actually blowing it. Then the Gemara backs out and concludes that the halacha of not blowing the shofar on Shabbos is rabbinic. (We are concerned that perhaps someone will forget and carry his shofar into the street on Shabbos.) Once the gemara comes to the conclusion that this din is only derabanan, how do we deal with the apparent discrepancy between the two pesukim describing Rosh Hashana as both a day of blowing shofar as well as a day of merely speaking about the shofar? The gemara never gives an alternate explanation to this apparent contradiction. One of the earlier achronim suggested that although this din is only rabbinic in origin, the Chumash is alluding to it because this gezeira d'rabanan existed already at the time the Chumash was written; Moshe Rabbeinu was the one who initiated it!
The Ba'al Hatanya in his collection of droshos for Rosh Hashana rejects this idea. He writes that it is well known that most of the gezeiros d'rabanan were instituted during the period of the second Beis Hamikdash. Based on kaballah sources he develops the idea that when the Jewish people have an independent Jewish government we are less concerned about the soton getting us to sin. During the period of the first Beis Hamikdash there was always a Jewish government. The Anshei Kenesses Hagedolah was a body that consisted of one hundred and twenty talmedei chachomim which was founded at the beginning of the second Beis Hamikdash. Because for several centuries (until the rebellion of the chashmonaim) there was no independent Jewish government, they felt it was imperative to introduce many seyagim and harchokas.
Many have the attitude that it is not necessary, and perhaps even improper, to introduce additional gezeiros today. They reason that whatever the rabbis of the Talmud did not prohibit is permitted and ought to remain so. The Mesilas Yesharim does not agree with this position. He assumes that this concluding passuk in parshas Acharei Mos is not directed only to the beis din hagadol or the rabbis of the Talmud, rather it is addressing each and every Jew! Whoever senses that he runs the risk of violating a Torah prohibition ought to accept upon himself a seyag to distance himself from the potential violation. The gezeiros recorded in the Talmud were intended for all Jews in all generations and in all societies. But if one senses that due to his profession, his society, etc. he particularly is at risk of violating some prohibition, he is instructed by the Torah to introduce some personal harchoka.
The rishonim point out that there was no concern of bal tosif in introducing all of the seyagim d'rabanan since the Torah itself encouraged us to do so. However, if the chachomim would add a gezeira l'gezeira, being concerned for the risk that someone may mistakenly violate a rabbinic law, since this was not warranted by the Torah, this would pose a problem of bal tosif (Pri Megadim.) To introduce a gezeira you must sense a significant need. If the risk of violating a Bibilical prohibition is far-fetched, we assume that the rabanan have no right to introduce a gezeira because of a milsa d'lo she'chi'ach. Similarly, the concern that someone may violate a rabbinic prohibition is considered insufficient cause to introduce a gezeira. Even if there is concern that one will fail to fulfill a Biblical mitzvah, but not that he will violate a Biblical prohibition, the Pri Megadim quotes Tosafos' interpretation (Menachos 68b s.v. v'ha kesiv) that this too is insufficient cause to introduce a gezeira, and such a gezeira would constitute a violation of bal tosif.