I remember reading something about a method called "hypergravity training" where an athlete would wear a weight vest ALL DAY LONG. Only removing it when going to bed.
Hypergravity Training: Women's Track and Field
Sands, William A.; Poole, R. Craig; Ford, Heide R.; Cervantez, Robert D.; Irvin, Randy C.; Major, James A.
Journal of Strength & Conditioning Research. 10(1):30-34, February 1996
Eleven female collegiate track and field athletes completed a study of hypergravity training to improve jumping performance. Five experimental and 6 controls were quasi-randomly selected from this intact team. The experimental athletes wore weighted vests for 3 weeks from morning to evening. The training load was periodized via percentages of body mass. Both groups continued their normal training loads in practice and competed in their indoor season. Vertical jump was measured weekly using a VertecTM device. The experimental athletes' mean overall change in vertical jump was 5.0 cm (SD = 1.42 cm); the control group mean improved 1.4 cm (SD = 1.73 cm).
Adaptive response of human skeletal muscle to simulated hypergravity condition
C. BOSCO Acta Physiologica Scandinavica. Volume 124, Issue 4, pages 507â??513, August 1985
(...) However, after that the athletes underwent a simulated 3 weeks high-gravity period. The hypergravitational condition was created by wearing a special vest filled with extra loads (11% of BW). The vest was used from morning to evening. No changes in the ordinary training programme were allowed. After the simulated high-gravity conditioning period significant improvement in almost all the variables studied was observed (P <0.05-0.001). Vertical jump performance was enhanced from 44.3 to 54.9 cm. The F-V curve remained stable all year but after hypergravity period shifted markedly to the right. The drastic improvement was attributed to be caused by a fast adaptation to the new functional requirements (1.1 g). Therefore, once the biological adaptation occurred the mechanical behaviour of the athlete's leg extensor muscle was similar to that which could be experienced in a field at low gravity condition (0.9 g). Adaptive response to the hypergravity conditioning was speculated to occur mainly at neurogenic level and less in myogenic component.
So although it is impractical (wearing a vest all day long for 3 weeks can look weird and be cumbersome) it seems to work. I never tried it myself, but always wanted to do it.